feat(dot_alfred/v2): bump SCHEMA_VERSION to 2 — added_at on MovieRelease

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.

tests/infrastructure/persistence/dot_alfred/v2/test_round_trip.py

@@ -17,6 +17,7 @@ from alfred.infrastructure.persistence.dot_alfred.v2.bridge import (
     series_release_to_sidecar,
 )
 from alfred.infrastructure.persistence.dot_alfred.v2.sidecar_release import (
+    SCHEMA_VERSION,
     MovieReleaseSidecar,
     SeriesReleaseSidecar,
 )
@@ -25,7 +26,7 @@ from alfred.infrastructure.persistence.dot_alfred.v2.sidecar_release import (
 class TestSeriesReleaseRoundTrip:
     def test_domain_to_sidecar_preserves_top_level(self, foundation_release):
         sidecar = series_release_to_sidecar(foundation_release)
-        assert sidecar.schema_version == 1
+        assert sidecar.schema_version == SCHEMA_VERSION
         assert sidecar.tmdb_id == 84958
         assert sidecar.imdb_id == "tt0804484"
         assert len(sidecar.releases) == 2
@@ -71,7 +72,7 @@ class TestSeriesReleaseRoundTrip:
 class TestMovieReleaseRoundTrip:
     def test_domain_to_sidecar_preserves_top_level(self, inception_release):
         sidecar = movie_release_to_sidecar(inception_release)
-        assert sidecar.schema_version == 1
+        assert sidecar.schema_version == SCHEMA_VERSION
         assert sidecar.tmdb_id == 27205
         assert sidecar.imdb_id == "tt1375666"
         assert sidecar.folder == "Inception.2010.1080p.BluRay.x264-GROUP"
@@ -89,3 +90,10 @@ class TestMovieReleaseRoundTrip:
         forced = restored.tracks.subtitle_tracks[1]
         assert forced.is_forced is True
         assert forced.language == "fre"
+
+    def test_added_at_round_trips_through_yaml(self, inception_release):
+        sidecar = movie_release_to_sidecar(inception_release)
+        text = yaml.safe_dump(sidecar.model_dump(mode="json"))
+        reloaded = MovieReleaseSidecar.model_validate(yaml.safe_load(text))
+        restored = movie_release_from_sidecar(reloaded)
+        assert restored.added_at == inception_release.added_at