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/lib/api/plex.dom.test.ts

@@ -0,0 +1,66 @@
+// @vitest-environment jsdom
+import { describe, it, expect } from "vitest";
+import {
+  coercePlexFilters,
+  plexFiltersSignature,
+  EMPTY_PLEX_FILTERS,
+  type PlexFilters,
+} from "./plex";
+
+describe("coercePlexFilters", () => {
+  it("migrates the legacy single collection/collectionTitle into the multi-select shape", () => {
+    const out = coercePlexFilters({ collection: "A", collectionTitle: "Marvel" });
+    expect(out.collections).toEqual(["A"]);
+    expect(out.collectionTitles).toEqual({ A: "Marvel" });
+  });
+
+  it("migrates a legacy collection with no title", () => {
+    const out = coercePlexFilters({ collection: "A" });
+    expect(out.collections).toEqual(["A"]);
+    expect(out.collectionTitles).toBeUndefined();
+  });
+
+  it("leaves an already-multi collections blob untouched (guard on !collections.length)", () => {
+    const out = coercePlexFilters({ collections: ["X", "Y"], collection: "A" });
+    expect(out.collections).toEqual(["X", "Y"]);
+  });
+
+  it("fills every default for an empty blob", () => {
+    expect(coercePlexFilters({})).toEqual(EMPTY_PLEX_FILTERS);
+    expect(coercePlexFilters({}).collections).toEqual([]);
+  });
+
+  it("drops the legacy sortDir field", () => {
+    const out = coercePlexFilters({ sortDir: "desc", genres: ["Drama"] });
+    expect((out as unknown as Record<string, unknown>).sortDir).toBeUndefined();
+    expect(out.genres).toEqual(["Drama"]);
+  });
+});
+
+describe("plexFiltersSignature", () => {
+  const base = (): PlexFilters => ({
+    ...EMPTY_PLEX_FILTERS,
+    genres: ["Drama"],
+    collections: ["A"],
+  });
+
+  it("is independent of key order for equivalent filters", () => {
+    // Same field values, keys assigned in a different literal order — appendPlexFilters emits by field,
+    // not by object key order, so the signatures must match.
+    const a: PlexFilters = { ...EMPTY_PLEX_FILTERS, genres: ["Drama"], collections: ["A"] };
+    const b: PlexFilters = { ...EMPTY_PLEX_FILTERS, collections: ["A"], genres: ["Drama"] };
+    expect(plexFiltersSignature(a)).toBe(plexFiltersSignature(b));
+  });
+
+  it("ignores display-only collectionTitles (not part of the wire params)", () => {
+    const a = base();
+    const b: PlexFilters = { ...base(), collectionTitles: { A: "Marvel" } };
+    expect(plexFiltersSignature(a)).toBe(plexFiltersSignature(b));
+  });
+
+  it("differs when a real filter differs", () => {
+    const a = base();
+    const b: PlexFilters = { ...base(), genres: ["Comedy"] };
+    expect(plexFiltersSignature(a)).not.toBe(plexFiltersSignature(b));
+  });
+});