fix(plex): resolve deep-review findings on the filter-bar overhaul

Self-review of the branch diff (feature/plex-filter-bar-overhaul) against main:

- Surprise rail query key omitted `lang` while the backend /surprise now filters
  against the language-specific genre column → a HU/EN switch under an active genre
  filter left the rail stale. Key it by scope+lang+filters, matching plexDiscover.
- Chip-removing a collection left its entry in `collectionTitles` (hashed into the
  plex query keys), splitting the cache from the pristine no-collection state and
  growing the titles map unboundedly. Prune the title on remove, like CollectionPicker.
- The saved-views default-mirror effect gained `cfg` in its deps (re-memoized on every
  live filter edit) → a synchronous localStorage write per keystroke. Depend on the
  module-stable mirror fn instead.
- Collections-browse search input had no accessible name (placeholder only). Add
  aria-label + type="search".
- Extract the tile hue-hash + gradient duplicated between PlexDiscover and
  plexBrowseLists into a shared lib/hueTile so the two surfaces can't drift.
- Document migration 0073 downgrade() as lossy and de-dup non-feed rows before
  recreating the narrow (user_id, name) constraint (verified by the drift test).
- Add unit tests for coercePlexFilters (legacy single-collection migration) and
  plexFiltersSignature (order-independence, title-ignoring).

Gate: tsc/lint/knip/prettier clean, 410 vitest; backend ruff + migration drift +
saved-views tests green.

frontend/src/components/PlexDiscover.tsx

@@ -4,6 +4,7 @@ import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { RotateCcw } from "lucide-react";
 import { qk } from "../lib/queryKeys";
 import { api, type PlexCard, type PlexDiscover as PlexDiscoverData } from "../lib/api";
+import { hueOf, hueTileGradient } from "../lib/hueTile";
 import { plexLang } from "../i18n";
 import { toggleWatchState } from "../lib/plexWatch";
 import { usePlex } from "./PlexProvider";
@@ -18,14 +19,6 @@ import { LoadingState, StateMessage } from "./QueryState";
 // info / watched-toggle handlers come from the shell (PlexBrowse) so a rail card behaves exactly like
 // a grid card; the watched-toggle optimistically flips THIS surface's cache too.
 
-// A stable, pleasant hue per genre string (no art to collage yet — colored tiles stand in). Hashing
-// keeps a genre the same color across renders/sessions without storing anything.
-function genreHue(name: string): number {
-  let h = 0;
-  for (let i = 0; i < name.length; i++) h = (h * 31 + name.charCodeAt(i)) % 360;
-  return h;
-}
-
 function CardRail({
   title,
   ariaLabel,
@@ -99,7 +92,7 @@ export default function PlexDiscover({
   });
   // Surprise-me is its own query so the ↻ button can redraw it without refetching the whole surface.
   const surpriseQ = useQuery({
-    queryKey: qk.plexSurprise(scope, filters),
+    queryKey: qk.plexSurprise(scope, lang, filters),
     queryFn: () => api.plexSurprise(scope, undefined, filters, lang),
     staleTime: 0,
   });
@@ -124,7 +117,7 @@ export default function PlexDiscover({
             : old,
         );
         qc.setQueryData<{ scope: string; items: PlexCard[] }>(
-          qk.plexSurprise(scope, filters),
+          qk.plexSurprise(scope, lang, filters),
           (old) => (old ? { ...old, items: flip(old.items) } : old),
         );
       },
@@ -207,16 +200,14 @@ export default function PlexDiscover({
           <h2 className="mb-3 text-sm font-semibold">{t("plex.discover.byGenre")}</h2>
           <div className="grid grid-cols-2 gap-3 sm:grid-cols-3 md:grid-cols-4 lg:grid-cols-6">
             {d.genres.map((g) => {
-              const hue = genreHue(g.value);
+              const hue = hueOf(g.value);
               return (
                 <button
                   key={g.value}
                   type="button"
                   onClick={() => onGenre(g.value)}
                   className="glass-hover relative flex h-20 items-end overflow-hidden rounded-xl p-3 text-left transition"
-                  style={{
-                    background: `linear-gradient(135deg, hsl(${hue} 55% 32%), hsl(${(hue + 40) % 360} 60% 22%))`,
-                  }}
+                  style={{ background: hueTileGradient(hue) }}
                 >
                   <span className="text-sm font-semibold text-white drop-shadow">{g.value}</span>
                   <span className="absolute right-2 top-2 text-2xs text-white/70">{g.count}</span>