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/tests/test_plex_detail.py

@@ -57,9 +57,11 @@ def test_rich_detail_prefers_imdb_entry_and_builds_link():
     assert d["cast"][1]["thumb"] is None  # no CDN headshot → not proxied
 
 
-def test_rich_detail_falls_back_to_audience_rating():
+def test_rich_detail_is_imdb_only_no_audience_fallback():
+    # E5 S6: imdb_rating is IMDb-ONLY. With no imdb:// Rating entry it's NULL — the Plex audience score
+    # is NOT mislabelled as an IMDb score (it's served separately via the `rating` column).
     d = detail.rich_detail({"audienceRating": 7.25})
-    assert d["imdb_rating"] == 7.2  # rounded to 1dp
+    assert d["imdb_rating"] is None
     assert d["imdb_id"] is None and d["imdb_url"] is None