Add collection/franchise ingestion

Collections were bare stubs (name only) lazily created the moment a
movie's belongs_to_collection reference was seen. Adds a real
ingest_collection pipeline: TMDb's /collection/{id} for overview and
poster/backdrop artwork, plus ordered CollectionMovie membership --
linking only movies already ingested into Aurora, the same
never-fabricate-a-stub rule movie recommendations already follow.

get_or_create_collection now refreshes name/overview on an existing
stub instead of only creating, guarded so a bare movie-stub call
(overview=None) never clobbers real data a full ingestion already
filled in.

New POST /api/v1/admin/ingest/collection endpoint and
ingest_collection_task; GET /api/v1/collections/{id} needed no changes
since its read path (collection_service.py) already assembled the full
detail shape.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

app/services/metadata/normalizer.py

@@ -138,6 +138,61 @@ class NormalizedMovie:
     recommendation_external_ids: list[str] = field(default_factory=list)
 
 
+@dataclass
+class NormalizedCollection:
+    external_id: str
+    name: str
+    overview: str | None
+    artwork: list[NormalizedArtwork] = field(default_factory=list)
+    # TMDb's "parts", ordered by release date rather than TMDb's own array
+    # order (which isn't guaranteed to be chronological) -- see
+    # normalize_collection.
+    part_external_ids: list[str] = field(default_factory=list)
+
+
+def normalize_collection(raw: dict) -> NormalizedCollection:
+    artwork: list[NormalizedArtwork] = []
+    if raw.get("poster_path"):
+        artwork.append(
+            NormalizedArtwork(
+                "poster",
+                _image_url_required(raw["poster_path"]),
+                raw["poster_path"],
+                None,
+                None,
+                None,
+                None,
+                is_primary=True,
+            )
+        )
+    if raw.get("backdrop_path"):
+        artwork.append(
+            NormalizedArtwork(
+                "backdrop",
+                _image_url_required(raw["backdrop_path"]),
+                raw["backdrop_path"],
+                None,
+                None,
+                None,
+                None,
+                is_primary=True,
+            )
+        )
+
+    parts = sorted(
+        raw.get("parts") or [],
+        key=lambda p: _parse_date(p.get("release_date")) or date.max,
+    )
+
+    return NormalizedCollection(
+        external_id=str(raw["id"]),
+        name=raw["name"],
+        overview=raw.get("overview") or None,
+        artwork=artwork,
+        part_external_ids=[str(p["id"]) for p in parts],
+    )
+
+
 _STATUS_MAP = {
     "Rumored": "rumored",
     "Planned": "planned",