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

tests/test_api_artists_create.py

@@ -72,3 +72,16 @@ async def test_patch_rename_validation(client):
         f"/api/artists/{created['id']}", json={})).status_code == 400
     assert (await client.patch(
         "/api/artists/999999", json={"name": "Ghost"})).status_code == 404
+
+
+@pytest.mark.asyncio
+async def test_names_lists_every_artist_alphabetically(client):
+    """The Latest feed's artist filter shows this list before anything is typed.
+    `autocomplete` stays empty for an empty query; this is the full list."""
+    for name in ("zed", "Alice", "bob"):
+        await client.post("/api/artists", json={"name": name})
+    resp = await client.get("/api/artists/names")
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert [a["name"] for a in body] == ["Alice", "bob", "zed"]
+    assert set(body[0]) == {"id", "name", "slug"}