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/db/src/types.ts

@@ -100,6 +100,7 @@ export type ContentUnitContentType =
 export type ContentUnitSegment = "full" | "upper" | "middle" | "lower" | "unknown";
 export type ContentUnitStatus = "provisional" | "confirmed" | "retired";
 export type ContentUnitEpisodeTotalSource = "tmdb" | "injected" | "manual" | "covered_floor" | "previous";
+export type ContentUnitSource = "release" | "file" | "tmdb" | "user";
 export type DecisionKind =
   | "identity_ambiguous"
   | "download_stalled"
@@ -642,6 +643,7 @@ export interface ContentUnitTable {
   episode_total_source: ContentUnitEpisodeTotalSource | null;
   episode_total_locked: Generated<boolean>;
   episode_total_at: Timestamptz | null;
+  source: ContentUnitSource | null;
 }
 
 export interface DecisionTable {