fix(plex): address S6 code-review — 5 findings

1. (correctness) Show hero rating regression: PlexBrowse's show hero rendered a
   rating only when show.imdb_rating was set, so after the S6 IMDb-only change a
   show with no imdb:// entry showed no rating. Now falls back to the audience
   `rating` like PlexInfo.
2. (correctness) ratingMin column mismatch: the rating filter and the /facets
   rating_max now key off coalesce(imdb_rating, rating) — the SAME "best score"
   the badge displays — so a ratingMin picked from a shown IMDb score narrows the
   right column instead of the raw audience `rating`.
3. (altitude) HU search gap: new query.vector_match() matches EITHER vector in HU
   (or_(search_vector_hu, search_vector)) so a row with no HU twin stays findable
   by its English title; ranking still uses the HU vector. field_recheck unions
   the HU+EN per-field tsvectors in HU, staying a strict subset of the prefilter.
4. (test-coverage) Extracted the badge logic to lib/plexRating.pickRatingBadge
   (shared by PlexInfo + the show hero) with a unit test (imdb-preferred, audience
   fallback, 0-score, none). Backend tests added for the HU union search + the
   IMDb-preferred rating filter.
5. (efficiency) Language switch no longer double-fetches the grid: browseQ is
   gated on a committedLang state that lags `lang` by one render, so the
   genre-clear effect settles into a single fetch instead of one throwaway fetch
   with stale-language genres plus a second.

DB-lane suite 295 passed / 2 skip; FE tsc/knip/lint clean, 217 vitest pass.

frontend/src/components/PlexInfo.tsx

@@ -5,6 +5,7 @@ import { useQueryClient } from "@tanstack/react-query";
 import { Check, ExternalLink, FolderPlus, ListPlus, Play, RotateCcw, Star, X } from "lucide-react";
 import { api, type PlexFilters, type PlexItemDetail } from "../lib/api";
 import { formatRuntime } from "../lib/format";
+import { pickRatingBadge } from "../lib/plexRating";
 import { notifyActionError } from "../lib/actionErrors";
 import {
   DetailCustomizeMenu,
@@ -198,9 +199,9 @@ export default function PlexInfo({
                 {(() => {
                   // IMDb score when we have one (with the IMDb link); otherwise the Plex audience
                   // score (no IMDb link/branding, so a non-IMDb score is never mislabelled as IMDb).
-                  const score = detail.imdb_rating ?? detail.rating;
-                  if (score == null) return null;
-                  const isImdb = detail.imdb_rating != null;
+                  const badge = pickRatingBadge(detail.imdb_rating, detail.rating);
+                  if (!badge) return null;
+                  const { score, isImdb } = badge;
                   return (
                     // decorative rating gold — intentionally NOT a status token
                     <span className="inline-flex items-center gap-1 rounded-md bg-amber-400/15 px-1.5 py-0.5 font-semibold text-amber-500">