feat(services): Slice B.2 dual-write — RuleEditorService writes deadline_rules AND procedural_events / sequencing_rules / legal_sources (t-paliad-305 / m/paliad#93)

Keeps the parallel new tables (mig 136, Slice B.1) in lock-step with
the legacy paliad.deadline_rules table through every write path on
RuleEditorService. Read paths stay on deadline_rules in B.2 — B.3
flips them and stops legacy writes.

* internal/services/dual_write.go (new) —
  - syncDualWriteFromDeadlineRule(ctx, tx, id): idempotent UPSERT of
    legal_sources + procedural_events + sequencing_rules from the
    just-written deadline_rules row. Pure SQL projection, no Go-side
    struct mapping. Synthetic-code mint expression is byte-identical
    to mig 136 ('null.' || first 8 hex of stripped uuid).
  - syncDeadlineDualLinks(ctx, tx, deadlineID): mirrors a deadline's
    legacy rule_id back-link onto deadlines.procedural_event_id +
    sequencing_rule_id. Handles NULL rule_id naturally (collapses both
    new columns to NULL).
  - CheckDualWriteDrift(ctx, conn): nine read-only count queries +
    integrity joins. Returns DualWriteDriftReport. HasDrift() bool for
    log routing.
  - StartDualWriteDriftCheckLoop(ctx, conn, interval): goroutine ticker
    that runs CheckDualWriteDrift every `interval` (default 6h) for
    the lifetime of ctx. Clean run logs at INFO; drift at WARN with
    full report.

* internal/services/rule_editor_service.go —
  - Create / UpdateDraft / CloneAsDraft / Publish / flipLifecycle
    each call syncDualWriteFromDeadlineRule(ctx, tx, id) after the
    deadline_rules mutation, before tx.Commit. Publish syncs BOTH the
    published draft AND the cloned-from peer it just archived as a
    cascade. The audit_reason already set via setAuditReasonTx
    propagates to the new-table writes (same TX, same session).

* internal/services/rule_editor_orphans.go —
  - ResolveOrphan calls syncDeadlineDualLinks after UPDATE
    paliad.deadlines SET rule_id = $1, so the parallel new columns
    follow the legacy back-link.

* internal/services/deadline_service.go —
  - DeadlineService.Update calls syncDeadlineDualLinks when
    input.RuleSet is true (auto/custom rule swap from t-paliad-258).

* cmd/server/main.go —
  - Spawns StartDualWriteDriftCheckLoop alongside CalDAV sync and
    reminder scanner. Inherits bgCtx so the goroutine stops on
    SIGTERM. Interval 6h.

* internal/services/dual_write_test.go (new) —
  - TestDualWrite_RuleEditorLifecycle: Create → UpdateDraft → Publish
    → Archive, asserts the new tables mirror at each step. Final
    CheckDualWriteDrift returns zero drift.
  - TestDualWrite_SyntheticCodeForNullSubmission: rule created with
    submission_code=NULL gets a 'null.<8hex>' procedural_events row
    matching mig 136's mint expression byte-for-byte.

Scope decisions documented in the commit:

- B.2 keeps read paths on deadline_rules. paliadin's "Read paths fall
  back to legacy" reads as "reads stay on legacy as the safety net
  while drift-check validates the new tables". B.3 swaps reads to
  new tables only AND stops writing to deadline_rules — that's a
  separate slice per the design's §5.2/§5.3 split.

- B.2 does NOT modify submission_drafts, projection_service, the
  Fristenrechner calculator, the SubmissionVarsService, the
  Schriftsätze list query, or any other reader. They keep reading
  deadline_rules unchanged. The new tables are populated in parallel
  for B.3's cutover.

- Audit triggers on deadline_rules continue to fire as before. The
  new tables have no audit triggers yet (a later slice can add
  parallel audit rows once the new tables are authoritative).

- Drift-check uses default 6h interval — short enough that a broken
  dual-write surfaces within the same business day, long enough that
  the count-COUNTs don't churn the pool. Override via the caller in
  cmd/server.

Hard rules followed:
- audit_reason set on every TX before any deadline_rules mutation
  (existing pattern; new-table writes share the same reason).
- No destructive op (B.2 is strictly additive in behaviour).
- New helpers idempotent (UPSERT ON CONFLICT DO UPDATE) — safe to
  call twice, safe to re-run after a partial failure.

Build + vet clean. TestMigrations_NoDuplicateSlot passes.

cmd/server/main.go

@@ -12,6 +12,7 @@ import (
 	"strconv"
 	"strings"
 	"syscall"
+	"time"
 
 	// Embed Go's IANA tz database into the binary so time.LoadLocation works
 	// without OS tzdata. The runtime image (alpine) doesn't ship /usr/share/
@@ -337,6 +338,13 @@ func main() {
 			log.Printf("CalDAV start: %v", err)
 		}
 		reminderSvc.Start(bgCtx)
+		// Slice B.2 dual-write drift check (t-paliad-305 / m/paliad#93).
+		// Runs every 6 h while the new procedural_events / sequencing_rules /
+		// legal_sources tables shadow the legacy paliad.deadline_rules
+		// table. A clean run logs at INFO; drift logs at WARN with the
+		// full report so a broken dual-write surfaces before the next
+		// deploy.
+		services.StartDualWriteDriftCheckLoop(bgCtx, pool, 6*time.Hour)
 		go func() {
 			<-bgCtx.Done()
 			log.Println("background services: shutdown signal received")