Phase 3 prep: Movie aggregate is about to become TMDB-only (no
filesystem fields). added_at is a release-time observation, not a
TMDB-aggregate concern, so it moves to MovieRelease +
MovieReleaseSidecar.
- Add added_at: datetime (required) to MovieRelease with a
type-check in __post_init__.
- Add added_at: datetime (required) to MovieReleaseSidecar.
- Bump SCHEMA_VERSION 1 → 2 with a version-history note.
- Bridge round-trips added_at via Pydantic mode="json" (datetime
→ ISO 8601 string).
- Tests: update MovieRelease fixtures, add a validator test, add
an added_at round-trip test, switch hard-coded `1` assertions
to SCHEMA_VERSION for future-proofing.
No v1 sidecars in the wild yet — no migration code needed.