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.

backend/alembic/versions/0073_saved_view_source.py

@@ -29,6 +29,11 @@ def upgrade() -> None:
 
 
 def downgrade() -> None:
+    # LOSSY / effectively forward-only: upgrade lets a user hold a feed view and a plex view of the same
+    # name, so re-creating the narrow (user_id, name) constraint can raise IntegrityError on such a pair.
+    # De-dup by dropping the non-feed rows first (they can't exist in the pre-0073 schema anyway) so the
+    # narrow constraint re-applies cleanly for the drift test's downgrade→re-upgrade cycle.
+    op.execute("DELETE FROM saved_views WHERE source <> 'feed'")
     op.drop_constraint("uq_user_source_saved_view", "saved_views", type_="unique")
     op.create_unique_constraint("uq_user_saved_view", "saved_views", ["user_id", "name"])
     op.drop_column("saved_views", "source")