feat(plex): S6 backend — language-aware serving + IMDb-only rating

E5 S6 backend half (no migration — the *_hu / rich columns exist from S5a/S5b).

Language-aware serving (?lang=hu serves the HU overlay twin, EN fallback per row):
- query.py: column pickers title_col / genres_col / vector_col; field_recheck and
  meta_filter_conds take `lang`. DISPLAY/FILTER pickers coalesce(*_hu, *) (EN
  fallback); SEARCH pickers use the raw *_hu column mirroring the generated
  search_vector_hu (coalesce to '', not EN) so the field recheck stays a strict
  subset of the GIN vector (the S2 invariant). genres_col type_coerces the coalesce
  to JSONB so the GIN `.contains` survives.
- browse.py: `lang` param on /library, /facets, /item, /show (validated to en|hu).
  unified_library title/vector/rank/field-scope + meta filters are language-aware
  for both the movie and show branches; facets unnest the active-language genre
  vocabulary (jsonb_typeof guard kept); detail endpoints serve HU title/summary/
  genres/show_title with EN fallback. Episodes stay EN (not overlay-localized).

IMDb-only rating (folded-in S5b review #5):
- detail.py rich_detail: drop the audienceRating/rating fallback → imdb_rating is
  NULL when there's no imdb:// Rating entry (stop mislabelling a Plex audience score
  as IMDb). item_detail now also serves the `rating` column (audience score;
  show_detail already did) so the UI can show it when there's no IMDb score.

Tests: test_plex_lang.py (HU title serving + EN fallback, language-specific facet
vocabulary, genre-filter language matching, item_detail lang + rating split);
test_plex_detail updated for IMDb-only. DB-lane suite 293 passed / 2 skipped; ruff clean.

backend/app/plex/detail.py

@@ -13,7 +13,10 @@ def rich_detail(meta: dict) -> dict:
     """Pull the info-page extras out of a full Plex metadata dict: an IMDb score + link, content
     rating, genres, director(s), studio, tagline, and cast (name/role/photo). Best-effort — any
     missing field is just omitted."""
-    # Rating: prefer the explicit IMDb entry in the Rating array, else the generic audienceRating.
+    # Rating: IMDb-ONLY — the explicit `imdb://` entry in the Rating array, or NULL. We deliberately do
+    # NOT fall back to audienceRating/rating here: that is a Plex audience score, not an IMDb score, and
+    # labelling it "imdb_rating" mislabels it. The audience score is served separately (item/show detail
+    # expose the `rating` column) so the UI can show it when there's no IMDb score.
     imdb_rating = None
     for r in meta.get("Rating") or []:
         if str(r.get("image") or "").startswith("imdb://"):
@@ -22,11 +25,6 @@ def rich_detail(meta: dict) -> dict:
             except (TypeError, ValueError):
                 pass
             break
-    if imdb_rating is None:
-        try:
-            imdb_rating = round(float(meta.get("audienceRating") or meta.get("rating")), 1)
-        except (TypeError, ValueError):
-            imdb_rating = None
     # IMDb id (tt…) from the Guid list → a clickable link.
     imdb_id = None
     for g in meta.get("Guid") or []: