Expose certifications and per-country release dates

TMDb's release_dates/content_ratings blocks were already fetched via
append_to_response but discarded at normalization -- every downstream
layer (ingestion write, repository read, schema, service) was unbuilt.
Adds the full pipeline: NormalizedReleaseDate + get_or_create_certification,
a shared polymorphic sync/read (movie and series both use it), and
MovieDetailOut/SeriesDetailOut certification + release_dates fields.

Series content_ratings has no date of its own, so each rating is anchored
to the show's first_air_date with release_type="tv".

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

app/services/common.py

@@ -6,13 +6,16 @@ from app.core.ids import aurora_id
 from app.models.artwork import Artwork
 from app.models.credit import Credit
 from app.models.enums import ArtworkType
+from app.models.movie import ReleaseDate
 from app.models.video import Video
 from app.schemas.common import (
     ArtworkOut,
     CastCreditOut,
+    CertificationOut,
     CrewCreditOut,
     ImagesOut,
     PersonBriefOut,
+    ReleaseDateOut,
     VideoOut,
 )
 
@@ -70,6 +73,37 @@ def to_video_out(video: Video) -> VideoOut:
     )
 
 
+def to_release_date_out(release_date: ReleaseDate) -> ReleaseDateOut:
+    certification = None
+    if release_date.certification is not None:
+        certification = CertificationOut(
+            system=release_date.certification.system, rating=release_date.certification.rating
+        )
+    return ReleaseDateOut(
+        country_code=release_date.country_code,
+        release_date=release_date.release_date,
+        release_type=release_date.release_type,
+        certification=certification,
+        note=release_date.note,
+    )
+
+
+def pick_certification(release_dates: list[ReleaseDate], *, preferred_country: str = "US") -> str | None:
+    """The single rating a client wants when it isn't picking a country
+    itself -- TMDb's own site defaults to the US rating badge, so this
+    does too, falling back to the first country that has one at all."""
+
+    fallback: str | None = None
+    for rd in release_dates:
+        if rd.certification is None:
+            continue
+        if rd.country_code == preferred_country:
+            return rd.certification.rating
+        if fallback is None:
+            fallback = rd.certification.rating
+    return fallback
+
+
 def split_credits(
     credits_: list[Credit], profile_paths: dict | None = None
 ) -> tuple[list[CastCreditOut], list[CrewCreditOut]]: