feat(identity): content unit data hygiene — specials, provisional promotion, duplicate merge (DH-1)

- metadata.includeSpecials (default true): full refresh and the language
  backfill fetch TMDB season 0 (specials) when present, like MP; season 0
  stays out of the episode-total ledger
- refreshMetadata promotes provisional season/episode units that match the
  TMDB season/episode layout to confirmed (source=tmdb), keeping ids and all
  references; non-matching units stay provisional
- content_unit.source (migration 0053): release / file / tmdb / user
- duplicate units (same entity/kind/season/episode/content type/segment) are
  merged as a refresh step and via `scripts/identity-cleanup.ts
  --merge-duplicates` (dry-run by default); keeper = most referenced, then
  confirmed, then earliest; references auto-discovered via FKs and repointed
- ensureUnits takes a per-unit advisory xact lock to stop concurrent lazy
  creation from producing duplicates
- runbooks updated; user DB read-only dry-run: 征途 S01E09/E10 4 units each
  -> 1 each (6 removed), not applied

Co-Authored-By: Claude Opus 5.5 <[email protected]>

packages/contracts/test/db-sync.test.ts

@@ -283,6 +283,8 @@ const ENUMS: Record<string, z.ZodEnum> = {
   BrowserControlLeaseStatus: C.BrowserLeaseStatus,
   // W5-C(0025)
   ContentUnitEpisodeTotalSource: C.EpisodeTotalSource,
+  // DH-1(0053)
+  ContentUnitSource: C.ContentUnitSource,
 };
 
 describe("contracts ⇔ db 表结构同步", () => {