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

frontend/test/storeUsage.spec.js

@@ -0,0 +1,63 @@
+// Reads of a store property that does not exist.
+//
+// JavaScript returns `undefined` for a missing property instead of failing, and
+// the frontend CI has no type-checker to catch it. `platformsStore.platforms`
+// was read for months by the Latest feed's filter bar and the Artists view. The
+// store exposes `list` and `byKey`, so both platform dropdowns opened empty and
+// only someone clicking them noticed.
+//
+// This scans the source for `platformsStore.<name>` and checks each name
+// against what the store really defines.
+
+import { readdirSync, readFileSync, statSync } from 'node:fs'
+import { join } from 'node:path'
+import { fileURLToPath } from 'node:url'
+
+import { createPinia, setActivePinia } from 'pinia'
+import { describe, expect, it } from 'vitest'
+
+import { usePlatformsStore } from '../src/stores/platforms.js'
+
+const SRC = fileURLToPath(new URL('../src', import.meta.url))
+const READ = /\bplatformsStore\.([A-Za-z_$][\w$]*)/g
+
+function sourceFiles (dir) {
+  return readdirSync(dir).flatMap((name) => {
+    const path = join(dir, name)
+    if (statSync(path).isDirectory()) return sourceFiles(path)
+    return /\.(vue|js)$/.test(name) ? [path] : []
+  })
+}
+
+function unknownReads (text, known) {
+  return [...text.matchAll(READ)].map(m => m[1]).filter(name => !known.has(name))
+}
+
+function storeKeys () {
+  setActivePinia(createPinia())
+  return new Set(Object.keys(usePlatformsStore()))
+}
+
+describe('platforms store usage', () => {
+  it('every platformsStore.<name> read in src names something the store defines', () => {
+    const known = storeKeys()
+    const offenders = sourceFiles(SRC).flatMap((file) =>
+      unknownReads(readFileSync(file, 'utf8'), known)
+        .map(name => `${file.slice(SRC.length + 1)}: platformsStore.${name}`),
+    )
+    expect(offenders).toEqual([])
+  })
+
+  it('the scan finds the read that shipped broken (positive control)', () => {
+    const known = storeKeys()
+    expect(known.has('list')).toBe(true)
+    expect(unknownReads('(platformsStore.platforms || []).map(p => p)', known))
+      .toEqual(['platforms'])
+  })
+
+  it('the scan actually walks the source tree', () => {
+    const hasRead = /\bplatformsStore\./
+    const withReads = sourceFiles(SRC).filter(f => hasRead.test(readFileSync(f, 'utf8')))
+    expect(withReads.length).toBeGreaterThan(2)
+  })
+})