feat(submissions): Composer Slice F — section reorder / hide / add custom (m/paliad#141)

The final Composer slice per design doc §12. Lawyer gains full
control over section composition: drag-and-drop reorder, per-section
delete, "+ Add section" picker for custom slugs that don't appear in
the base's default spec. Combined with Slice B's hide toggle, this
closes out the A→F sequence — Composer A→F is complete.

Backend (internal/services/submission_section_service.go, +120 LoC):

- SectionService.Create — adds a new section row to a draft. Validates
  section_key + labels + kind (must be prose/requests/evidence).
  Auto-assigns next order_index when OrderIndex=0; collisions on
  (draft_id, section_key) surface as ErrInvalidInput.

- SectionService.Delete — removes one section by id. Returns
  ErrSubmissionSectionNotFound when nothing was deleted.

- SectionService.Reorder — accepts a sequence of section_ids, rewrites
  every row's order_index to (1..N)×10 transactionally. Returns the
  refreshed list. Sections not present in the sequence are silently
  ignored (defensive — partial reorder doesn't lose rows).

Handlers (internal/handlers/submission_sections.go, +180 LoC):

- POST /api/submission-drafts/{draft_id}/sections — owner-scoped via
  SubmissionDraftService.Get. 400 on slug collision / invalid kind.
- DELETE /api/submission-drafts/{draft_id}/sections/{section_id} —
  owner + section-belongs-to-draft cross-check. 204 on success.
- POST /api/submission-drafts/{draft_id}/sections/reorder — accepts
  {"section_order": [uuid, uuid, ...]}; returns refreshed sections list.

Frontend (frontend/src/client/submission-draft.ts, +260 LoC):

- Each section row gains a drag handle (⋮⋮) on the left of the head.
  Drag handle is the only draggable element; contentEditable
  selections inside the editor body keep working. HTML5 native DnD,
  no library.
- Drop-target highlighting via .submission-draft-section--drop-target
  (border-top accent). Cleanup on dragend / drop / cancel.
- Per-section "Delete" button next to the existing Hide/Include
  toggle. Confirm prompt prevents accidental loss of typed prose.
- "+ Add section" trailing affordance below the section list opens an
  inline form (slug + DE label + EN label + kind dropdown). Submit
  POSTs to the new endpoint; on success splices the row into
  state.view.sections and re-paints.

CSS (frontend/src/styles/global.css, +65 LoC):

- .submission-draft-section-handle (grab cursor + hover background +
  active=grabbing).
- .submission-draft-section--dragging / --drop-target visual states.
- .submission-draft-add-section form layout (dashed border + lime
  primary submit).

Tests (internal/services/submission_section_slice_f_test.go, NEW,
TEST_DATABASE_URL-gated):
- Create custom section + slug-collision surface as ErrInvalidInput.
- Delete + repeat-delete returns ErrSubmissionSectionNotFound.
- Reorder reverses 10 seeded sections + verifies the resulting
  order_index sequence is ascending and matches the input order.

Build hygiene: go build/vet/test -short clean (all packages);
bun run build clean (2906 i18n keys, data-i18n scan clean).

Hard rules honoured:
- NO new migrations (Slice F is pure code on Slice A's schema).
- NO behavior change for pre-Composer drafts (no section rows → no
  drag handles to drag).
- {{rule.X}} aliases preserved (custom sections render through the
  same composer pipeline as default sections).
- Q2/Q9/Q10 ratifications preserved.

This closes the Composer slice sequence A → F. The full feature set
ratified by m on 2026-05-26 is now in place:
  A — base picker + read-only section list (mig 146/147/148)
  B — editable prose + anchor-spliced render + MD→OOXML walker
  C — building-blocks library + section picker (mig 149)
  D — rich prose (headings, lists, blockquote, hyperlinks)
  E — specialist bases lg-duesseldorf + upc-formal (mig 150)
  F — section reorder / delete / add custom

t-paliad-318 Slice F

internal/services/submission_section_service.go

@@ -178,6 +178,130 @@ func (s *SectionService) Update(ctx context.Context, sectionID uuid.UUID, patch
 	return &sec, nil
 }
 
+// SectionCreateInput is the payload for adding a new (lawyer-custom)
+// section to a draft (t-paliad-318 Slice F).
+type SectionCreateInput struct {
+	DraftID     uuid.UUID
+	SectionKey  string
+	Kind        string
+	LabelDE     string
+	LabelEN     string
+	ContentMDDE string
+	ContentMDEN string
+	OrderIndex  int  // 0 = append at end
+	Included    bool // defaults to true if not specified at the handler
+}
+
+// Create inserts a new section row for the draft. The section_key
+// must not already exist on this draft (UNIQUE constraint at the DB
+// catches collisions and surfaces as ErrInvalidInput).
+//
+// OrderIndex=0 means "auto-assign at the end" — the service queries
+// the current max(order_index) and increments. Non-zero values insert
+// at the requested position; the caller is responsible for any
+// subsequent Reorder if they intend to push existing rows down.
+func (s *SectionService) Create(ctx context.Context, in SectionCreateInput) (*SubmissionSection, error) {
+	in.SectionKey = strings.TrimSpace(in.SectionKey)
+	in.LabelDE = strings.TrimSpace(in.LabelDE)
+	in.LabelEN = strings.TrimSpace(in.LabelEN)
+	if in.SectionKey == "" || in.LabelDE == "" || in.LabelEN == "" {
+		return nil, ErrInvalidInput
+	}
+	switch in.Kind {
+	case "prose", "requests", "evidence":
+	default:
+		return nil, ErrInvalidInput
+	}
+
+	if in.OrderIndex == 0 {
+		var maxOrder int
+		err := s.db.GetContext(ctx, &maxOrder,
+			`SELECT COALESCE(MAX(order_index), 0) FROM paliad.submission_sections WHERE draft_id = $1`,
+			in.DraftID)
+		if err != nil {
+			return nil, fmt.Errorf("max order_index: %w", err)
+		}
+		in.OrderIndex = maxOrder + 1
+	}
+
+	var sec SubmissionSection
+	err := s.db.GetContext(ctx, &sec,
+		`INSERT INTO paliad.submission_sections
+		     (draft_id, section_key, order_index, kind,
+		      label_de, label_en, included,
+		      content_md_de, content_md_en)
+		 VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9)
+		 RETURNING `+sectionColumns,
+		in.DraftID, in.SectionKey, in.OrderIndex, in.Kind,
+		in.LabelDE, in.LabelEN, in.Included,
+		in.ContentMDDE, in.ContentMDEN)
+	if err != nil {
+		// UNIQUE (draft_id, section_key) collision → invalid input.
+		if strings.Contains(err.Error(), "unique") || strings.Contains(err.Error(), "23505") {
+			return nil, fmt.Errorf("%w: section_key already exists on this draft", ErrInvalidInput)
+		}
+		return nil, fmt.Errorf("create submission section: %w", err)
+	}
+	return &sec, nil
+}
+
+// Delete removes one section row by id. Owner-scope is the caller's
+// responsibility (the handler runs SubmissionDraftService.Get first).
+func (s *SectionService) Delete(ctx context.Context, sectionID uuid.UUID) error {
+	res, err := s.db.ExecContext(ctx,
+		`DELETE FROM paliad.submission_sections WHERE id = $1`,
+		sectionID)
+	if err != nil {
+		return fmt.Errorf("delete submission section: %w", err)
+	}
+	n, _ := res.RowsAffected()
+	if n == 0 {
+		return ErrSubmissionSectionNotFound
+	}
+	return nil
+}
+
+// Reorder updates the order_index of every section row for the draft
+// according to the supplied ID sequence. Transactional — partial
+// failures roll back. Any section_id present on the draft but not in
+// the sequence keeps its previous order_index, then sorts last by
+// updated_at (so a partial reorder doesn't lose rows the caller
+// forgot to mention).
+func (s *SectionService) Reorder(ctx context.Context, draftID uuid.UUID, order []uuid.UUID) ([]SubmissionSection, error) {
+	tx, err := s.db.BeginTxx(ctx, nil)
+	if err != nil {
+		return nil, fmt.Errorf("reorder tx: %w", err)
+	}
+	committed := false
+	defer func() {
+		if !committed {
+			_ = tx.Rollback()
+		}
+	}()
+
+	// Each id in order gets order_index 10, 20, 30, ... (gaps so a
+	// future single-row insert doesn't trigger a full reflow). Ids
+	// not present on the draft are silently ignored.
+	for i, sectionID := range order {
+		idx := (i + 1) * 10
+		_, err := tx.ExecContext(ctx,
+			`UPDATE paliad.submission_sections
+			    SET order_index = $1
+			  WHERE id = $2 AND draft_id = $3`,
+			idx, sectionID, draftID)
+		if err != nil {
+			return nil, fmt.Errorf("reorder update: %w", err)
+		}
+	}
+
+	if err := tx.Commit(); err != nil {
+		return nil, fmt.Errorf("commit reorder: %w", err)
+	}
+	committed = true
+
+	return s.ListForDraft(ctx, draftID)
+}
+
 // SeedFromSpec inserts one row per BaseSectionSpec.Default into
 // submission_sections for the given draft. Runs inside the caller's
 // transaction (the SubmissionDraftService.Create path wraps the

internal/services/submission_section_slice_f_test.go

@@ -0,0 +1,152 @@
+package services
+
+// Live-DB tests for Slice F section service additions (Create + Delete
+// + Reorder). Gated on TEST_DATABASE_URL, mirroring Slice A's pattern.
+
+import (
+	"context"
+	"os"
+	"testing"
+
+	"github.com/google/uuid"
+	"github.com/jmoiron/sqlx"
+	_ "github.com/lib/pq"
+
+	"mgit.msbls.de/m/paliad/internal/db"
+)
+
+func TestSectionService_SliceF(t *testing.T) {
+	url := os.Getenv("TEST_DATABASE_URL")
+	if url == "" {
+		t.Skip("TEST_DATABASE_URL not set — skipping live DB test")
+	}
+	if err := db.ApplyMigrations(url); err != nil {
+		t.Fatalf("apply migrations: %v", err)
+	}
+	pool, err := sqlx.Connect("postgres", url)
+	if err != nil {
+		t.Fatalf("connect: %v", err)
+	}
+	defer pool.Close()
+
+	ctx := context.Background()
+
+	bases := NewBaseService(pool)
+	sections := NewSectionService(pool)
+
+	// Seed user + draft so we have a draft_id to attach sections to.
+	userID := uuid.New()
+	cleanup := func() {
+		pool.ExecContext(ctx, `DELETE FROM paliad.submission_sections WHERE draft_id IN (SELECT id FROM paliad.submission_drafts WHERE user_id = $1)`, userID)
+		pool.ExecContext(ctx, `DELETE FROM paliad.submission_drafts   WHERE user_id = $1`, userID)
+		pool.ExecContext(ctx, `DELETE FROM paliad.users               WHERE id = $1`, userID)
+		pool.ExecContext(ctx, `DELETE FROM auth.users                 WHERE id = $1`, userID)
+	}
+	cleanup()
+	defer cleanup()
+
+	email := "slice-f-" + userID.String()[:8] + "@hlc.com"
+	if _, err := pool.ExecContext(ctx, `INSERT INTO auth.users (id, email) VALUES ($1, $2)`, userID, email); err != nil {
+		t.Fatalf("seed auth.users: %v", err)
+	}
+	if _, err := pool.ExecContext(ctx,
+		`INSERT INTO paliad.users (id, email, display_name, office, global_role, lang)
+		 VALUES ($1, $2, 'Slice F User', 'munich', 'standard', 'de')`,
+		userID, email); err != nil {
+		t.Fatalf("seed paliad.users: %v", err)
+	}
+	users := NewUserService(pool)
+	projects := NewProjectService(pool, users)
+	parties := NewPartyService(pool, projects)
+	vars := NewSubmissionVarsService(pool, projects, parties, users)
+	renderer := NewSubmissionRenderer()
+	drafts := NewSubmissionDraftService(pool, projects, vars, renderer)
+	drafts.AttachComposer(bases, sections, "HLC")
+
+	d, err := drafts.Create(ctx, userID, nil, "de.inf.lg.erwidg", "de")
+	if err != nil {
+		t.Fatalf("Create draft: %v", err)
+	}
+	initial, err := sections.ListForDraft(ctx, d.ID)
+	if err != nil {
+		t.Fatalf("ListForDraft initial: %v", err)
+	}
+	if len(initial) != 10 {
+		t.Fatalf("expected 10 seeded sections; got %d", len(initial))
+	}
+
+	t.Run("Create custom section", func(t *testing.T) {
+		created, err := sections.Create(ctx, SectionCreateInput{
+			DraftID:    d.ID,
+			SectionKey: "berufungsantraege",
+			Kind:       "requests",
+			LabelDE:    "Berufungsanträge",
+			LabelEN:    "Appeal requests",
+			Included:   true,
+		})
+		if err != nil {
+			t.Fatalf("Create: %v", err)
+		}
+		if created.OrderIndex <= 10 {
+			t.Errorf("auto-assigned order_index should be > existing max; got %d", created.OrderIndex)
+		}
+		// Slug collision must surface as ErrInvalidInput.
+		_, err = sections.Create(ctx, SectionCreateInput{
+			DraftID: d.ID, SectionKey: "berufungsantraege",
+			Kind: "prose", LabelDE: "x", LabelEN: "x", Included: true,
+		})
+		if err == nil {
+			t.Errorf("expected unique-key collision error; got nil")
+		}
+	})
+
+	t.Run("Delete section", func(t *testing.T) {
+		// Grab one of the seeded rows to delete.
+		current, _ := sections.ListForDraft(ctx, d.ID)
+		var victimID uuid.UUID
+		for _, s := range current {
+			if s.SectionKey == "exhibits" {
+				victimID = s.ID
+				break
+			}
+		}
+		if victimID == uuid.Nil {
+			t.Fatalf("expected exhibits section to exist")
+		}
+		if err := sections.Delete(ctx, victimID); err != nil {
+			t.Fatalf("Delete: %v", err)
+		}
+		// Second delete returns not-found.
+		if err := sections.Delete(ctx, victimID); err == nil {
+			t.Errorf("expected ErrSubmissionSectionNotFound on second delete")
+		}
+	})
+
+	t.Run("Reorder sections", func(t *testing.T) {
+		current, _ := sections.ListForDraft(ctx, d.ID)
+		if len(current) < 3 {
+			t.Skipf("need at least 3 sections to test reorder; got %d", len(current))
+		}
+		// Reverse the order list.
+		ids := make([]uuid.UUID, 0, len(current))
+		for i := len(current) - 1; i >= 0; i-- {
+			ids = append(ids, current[i].ID)
+		}
+		reordered, err := sections.Reorder(ctx, d.ID, ids)
+		if err != nil {
+			t.Fatalf("Reorder: %v", err)
+		}
+		// Verify the first ID in our list now has the lowest order_index.
+		if reordered[0].ID != ids[0] {
+			t.Errorf("first ID after reorder = %s; want %s", reordered[0].ID, ids[0])
+		}
+		// Order indices should be ascending.
+		prev := 0
+		for _, s := range reordered {
+			if s.OrderIndex <= prev {
+				t.Errorf("non-ascending order_index after reorder: %d (prev=%d) at %s", s.OrderIndex, prev, s.SectionKey)
+			}
+			prev = s.OrderIndex
+		}
+	})
+}