fix(e5): address code-review findings (TMDB guid media_type, search ingest guard, discover keying)

Follow-ups from the epic-end /code-review of the E5 branch:

- tmdb.resolve_recs + people._resolve_known_for: match recommendations/
  credits to the library bucketed by media_type. TMDB movie ids and tv ids
  are independent sequences, so a bare `tmdb:<id>` match could deep-link a
  movie rec to an unrelated show (or vice-versa) on a numeric collision.
  Regression test added (test_resolve_recs_matches_by_media_type_not_bare_id).
- routes/search: wrap the _ingest_candidates call so a DB/enrich failure
  degrades to a 422 with a message instead of a bare 500 hidden behind the
  FE's generic "search failed" (extends the B-core-1 rationale); rollback on
  failure. Stamp enriched_at with its own timestamp taken after the enrich,
  not the stub-insert placeholder.
- routes/plex/discover: key serialized cards back by rating_key (globally
  unique in Plex) instead of id(obj); fix the stale rows 3-tuple annotation.

backend/app/routes/plex/people.py

@@ -36,24 +36,32 @@ def _resolve_known_for(
     popularity order. Matched by the canonical guid `tmdb:<id>`; out-of-library credits are dropped."""
     if not known_for:
         return []
-    guids = [f"tmdb:{k['tmdb_id']}" for k in known_for if k.get("tmdb_id")]
-    if not guids:
+    # Bucket by media_type: TMDB movie ids and tv ids collide numerically, so a movie credit must only
+    # match a library movie and a tv credit only a library show (mirrors tmdb.resolve_recs). Matching
+    # blind to media_type would render a tv credit as an unrelated movie whose tmdb id happens to match.
+    movie_guids = [
+        f"tmdb:{k['tmdb_id']}" for k in known_for if k.get("tmdb_id") and k.get("media_type") != "tv"
+    ]
+    tv_guids = [
+        f"tmdb:{k['tmdb_id']}" for k in known_for if k.get("tmdb_id") and k.get("media_type") == "tv"
+    ]
+    if not movie_guids and not tv_guids:
         return []
     movies: dict[str, PlexItem] = {}
-    if movie_lib_ids:
+    if movie_lib_ids and movie_guids:
         for m in db.execute(
             select(PlexItem).where(
                 PlexItem.kind == "movie",
                 PlexItem.library_id.in_(movie_lib_ids),
-                PlexItem.guid.in_(guids),
+                PlexItem.guid.in_(movie_guids),
             )
         ).scalars():
             movies.setdefault(m.guid, m)
     shows: dict[str, PlexShow] = {}
-    if show_lib_ids:
+    if show_lib_ids and tv_guids:
         for s in db.execute(
             select(PlexShow).where(
-                PlexShow.library_id.in_(show_lib_ids), PlexShow.guid.in_(guids)
+                PlexShow.library_id.in_(show_lib_ids), PlexShow.guid.in_(tv_guids)
             )
         ).scalars():
             shows.setdefault(s.guid, s)
@@ -70,10 +78,12 @@ def _resolve_known_for(
     out: list[dict] = []
     for k in known_for:
         g = f"tmdb:{k.get('tmdb_id')}"
-        if g in movies:
+        # A credit resolves against its OWN media_type's bucket only.
+        if k.get("media_type") == "tv":
+            if g in shows:
+                out.append(show_card(shows[g], show_statuses.get(shows[g].id, "new")))
+        elif g in movies:
             out.append(movie_card(movies[g], m_states.get(movies[g].id)))
-        elif g in shows:
-            out.append(show_card(shows[g], show_statuses.get(shows[g].id, "new")))
     return out