refactor(plex): move content_libs down into the query layer

The browse scope is a pure DB query with no request state, but it lived in
routes/plex/_shared.py. E24's Home surface needs the same scope from a domain
package, and a domain package importing from routes/ inverts the one-way
dependency CLAUDE.md draws.

Moved to app/plex/query.py and renamed to content_libs (public — it is a domain
helper now). Swept all five call sites and both tests in one go rather than
leaving a re-export shim: the canonical-patterns table already names the
routes.admin re-export as the anti-pattern.

backend/app/routes/plex/_shared.py

@@ -3,13 +3,9 @@
 import logging
 
 from fastapi import HTTPException
-from sqlalchemy import select
 from sqlalchemy.orm import Session
 
-from app.models import (
-    PlexItem,
-    PlexLibrary,
-)
+from app.models import PlexItem
 from app.plex.lookups import by_rating_key
 
 log = logging.getLogger("siftlode.plex")
@@ -25,21 +21,3 @@ def item_or_404(db: Session, rating_key: str) -> PlexItem:
 def _norm_lang(lang: str | None) -> str:
     """The overlay has an EN base + an optional HU twin; anything other than `hu` serves EN."""
     return "hu" if lang == "hu" else "en"
-
-
-def _content_libs(db: Session) -> tuple[list[int], list[int]]:
-    """The BROWSABLE content libraries, partitioned into (movie_lib_ids, show_lib_ids) — the single
-    source of the browse scope. Excludes HU overlay libraries (`overlay_of` set): those are never
-    browsed on their own, they only lend their Hungarian metadata to the EN twin's rows (E5 S5)."""
-    libs = (
-        db.execute(
-            select(PlexLibrary).where(
-                PlexLibrary.enabled.is_(True), PlexLibrary.overlay_of.is_(None)
-            )
-        )
-        .scalars()
-        .all()
-    )
-    movie_lib_ids = [lb.id for lb in libs if lb.kind == "movie"]
-    show_lib_ids = [lb.id for lb in libs if lb.kind == "show"]
-    return movie_lib_ids, show_lib_ids