fix: the Latest feed's filter dropdowns opened empty

Operator: "the filters in the latest feed, drop down but don't have values". Two separate causes:

- Platform: PostsFilterBar built its items from `platformsStore.platforms`. The platforms store has never had that property; it exposes `list` and `byKey`. The read returned undefined, `|| []` turned that into an empty list, and nothing failed. ArtistsView had copied the same read, so the Browse → Artists platform filter was empty too. Both now read `list` and show platform names rather than raw keys.

- Artist: the autocomplete searched the server only after something was typed (autocomplete returns [] for an empty query by design, which its tests pin). Opening the dropdown therefore showed an empty menu. PostsFilterBar now loads every artist once from a new lightweight `GET /api/artists/names` (id, name, slug; alphabetical; no joins) and filters client-side, so the list is there on open. A deep-linked artist_id now also shows the artist's real name instead of "Artist #id".

Guard: frontend/test/storeUsage.spec.js scans src for `platformsStore.<name>` and fails on any name the store doesn't define, since the frontend CI has no type-checker to catch this. A positive control shows the shipped `platformsStore.platforms` read is flagged, and a vacuity check confirms the scan really walks the tree.

tests/test_api_artists_create.py covers /names.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SHQB1YukL3VyvMK8rcbmV9

backend/app/api/artists.py

@@ -65,6 +65,16 @@ async def autocomplete():
     ])
 
 
+@artists_bp.route("/names", methods=["GET"])
+async def names():
+    """Every artist, id + name + slug, alphabetical. For filter pickers that
+    list artists before anything is typed; `autocomplete` deliberately returns
+    nothing for an empty query."""
+    async with get_session() as session:
+        rows = await ArtistService(session).all_names()
+    return jsonify([{"id": i, "name": n, "slug": s} for i, n, s in rows])
+
+
 @artists_bp.route("/directory", methods=["GET"])
 async def directory():
     """FC-3f: cursor-paginated artists directory.