fix(projects): three project-detail page hotfixes

m hit a cluster of three bugs on /projects/{id}/submissions:

1. 500 on /api/projects/{id}/partner-units — DerivationService.AttachedUnit
   scanned derive_unit_roles (text[]) into a plain []string. sqlx returns
   []uint8 for array columns without an adapter. Swap to pq.StringArray
   (same shape as the other array-scanned types in the codebase).

2. 404 on /projects/{id}/submissions — every other project-tab path
   (history, deadlines, team, checklists, …) is registered in handlers.go
   routing all to handleProjectsDetailPage so deep links work, but the
   submissions tab added in t-paliad-230 never got the matching route.
   Result: m navigates to the share-able URL and gets the 404 chrome.
   Add the missing route entry.

3. Create / update project rejected by projekte_client_number_check —
   the CHECK is `client_number IS NULL OR matches '^[0-9]{6}$'`, but the
   form sends empty string "" for an unset field. The Create path passed
   `*input.ClientNumber` raw; the Update path's appendSetSkippable did
   the same. Both now route through a new nullableTrimmed helper that
   coerces empty/whitespace to nil → SQL NULL → constraint accepts.
   matter_number gets the same treatment for symmetry.

Verified the SQL by EXPLAIN against the live DB on the today-filter
hotfix (becf4f0). These three fixes only change Go-side type / nil-
coercion, so no SQL-syntax exposure.

internal/handlers/handlers.go

@@ -466,6 +466,10 @@ func Register(mux *http.ServeMux, client *auth.Client, giteaAPIToken string, svc
 	protected.HandleFunc("GET /projects/{id}/notes", gateOnboarded(handleProjectsDetailPage))
 	protected.HandleFunc("GET /projects/{id}/checklists", gateOnboarded(handleProjectsDetailPage))
 	protected.HandleFunc("GET /projects/{id}/team", gateOnboarded(handleProjectsDetailPage))
+	// t-paliad-230 Schriftsätze tab — same shape as every other tab above.
+	// Without this route the deep-link 404s; the tab still works via
+	// in-page click since it just toggles a panel.
+	protected.HandleFunc("GET /projects/{id}/submissions", gateOnboarded(handleProjectsDetailPage))
 	// t-paliad-177 — standalone Project Timeline / Chart page (Slice 1).
 	// Horizontal SVG renderer mounted client-side; reuses the existing
 	// /api/projects/{id}/timeline JSON endpoint for data.

internal/services/derivation_service.go

@@ -42,12 +42,15 @@ func NewDerivationService(db *sqlx.DB, projects *ProjectService, partnerUnit *Pa
 // the configured derive_unit_roles. The frontend renders this on the
 // /projects/{id}/settings/team Partner Units section.
 type AttachedUnit struct {
-	ProjectID             uuid.UUID `db:"project_id"              json:"project_id"`
-	PartnerUnitID         uuid.UUID `db:"partner_unit_id"         json:"partner_unit_id"`
-	UnitName              string    `db:"unit_name"               json:"unit_name"`
-	DeriveUnitRoles       []string  `db:"derive_unit_roles"       json:"derive_unit_roles"`
-	DeriveGrantsAuthority bool      `db:"derive_grants_authority" json:"derive_grants_authority"`
-	DerivedMemberCount    int       `db:"derived_member_count"    json:"derived_member_count"`
+	ProjectID     uuid.UUID `db:"project_id"      json:"project_id"`
+	PartnerUnitID uuid.UUID `db:"partner_unit_id" json:"partner_unit_id"`
+	UnitName      string    `db:"unit_name"       json:"unit_name"`
+	// derive_unit_roles is a Postgres text[]; sqlx returns it as []byte
+	// without an array adapter, so we use pq.StringArray for the scan
+	// and convert to []string in JSON via a tiny ergonomics wrapper.
+	DeriveUnitRoles       pq.StringArray `db:"derive_unit_roles"       json:"derive_unit_roles"`
+	DeriveGrantsAuthority bool           `db:"derive_grants_authority" json:"derive_grants_authority"`
+	DerivedMemberCount    int            `db:"derived_member_count"    json:"derived_member_count"`
 }
 
 // DerivedMembership is one (unit, role) pair through which a user currently

internal/services/project_service.go

@@ -924,7 +924,7 @@ func (s *ProjectService) Create(ctx context.Context, userID uuid.UUID, input Cre
 		input.Title, input.Reference, input.Description, status,
 		userID,
 		input.Industry, input.Country, input.BillingReference,
-		input.ClientNumber, input.MatterNumber, input.NetDocumentsURL,
+		nullableTrimmed(input.ClientNumber), nullableTrimmed(input.MatterNumber), input.NetDocumentsURL,
 		input.PatentNumber, input.FilingDate, input.GrantDate,
 		input.Court, input.CaseNumber, input.ProceedingTypeID,
 		nullableOurSide(input.OurSide),
@@ -1038,10 +1038,13 @@ func (s *ProjectService) Update(ctx context.Context, userID, id uuid.UUID, input
 		appendSet("billing_reference", *input.BillingReference)
 	}
 	if input.ClientNumber != nil {
-		appendSetSkippable("client_number", *input.ClientNumber)
+		// Coerce empty string → NULL so the
+		// projekte_client_number_check ('^[0-9]{6}$' OR NULL) accepts a
+		// blanked field (m, 2026-05-22 — "I cant add a project").
+		appendSetSkippable("client_number", nullableTrimmed(input.ClientNumber))
 	}
 	if input.MatterNumber != nil {
-		appendSet("matter_number", *input.MatterNumber)
+		appendSet("matter_number", nullableTrimmed(input.MatterNumber))
 	}
 	if input.NetDocumentsURL != nil {
 		appendSet("netdocuments_url", *input.NetDocumentsURL)
@@ -2033,6 +2036,23 @@ func nullableInstanceLevel(p *string) any {
 	return s
 }
 
+// nullableTrimmed returns nil for an empty / whitespace value so the SQL
+// driver writes NULL, otherwise the trimmed string. Used for nullable
+// text columns whose constraints reject the empty string (e.g.
+// projekte_client_number_check requires NULL or 6 digits — a blank
+// client_number field on the project form would otherwise fail the
+// constraint instead of being treated as "not set").
+func nullableTrimmed(p *string) any {
+	if p == nil {
+		return nil
+	}
+	s := strings.TrimSpace(*p)
+	if s == "" {
+		return nil
+	}
+	return s
+}
+
 // nullableOurSide returns nil for an empty / whitespace value so the
 // SQL driver writes NULL, otherwise the trimmed string. Mirrors the
 // Update payload contract: empty string from the form clears the