Fix crash: watch_provider was never registered as an Aurora ID entity type

app/services/common.py:build_watch_providers_out calls
aurora_id("watch_provider", ...) for every provider entry -- this raised
ValueError on any movie/series detail response (or the dedicated
/watch-providers route) containing watch-provider data, since the entity
type was never added to ENTITY_PREFIXES. Never caught locally because
every test exercising this path is an integration test that skips
without a reachable Postgres.

Not yet deployed (redeploy has been on hold pending the bulk import), so
this never hit production traffic, but confirms it's worth actually
running the test suite against real Postgres before the next deploy
rather than trusting lint/mypy + locally-skipped tests alone.

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

app/core/ids.py

@@ -50,6 +50,7 @@ ENTITY_PREFIXES: dict[str, str] = {
     "country": "country",
     "alternate_title": "altt",
     "release": "rel",
+    "watch_provider": "watch_provider",
 }