feat(fc3c): downloads Pinia store + sources.checkNow action + vitests

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/test/sources.spec.js

@@ -78,4 +78,29 @@ describe('sources store', () => {
     expect(urls[0]).toContain('/api/artists/autocomplete')
     expect(urls[0]).toContain('q=al')
   })
+
+  it('checkNow posts to /api/sources/<id>/check', async () => {
+    const s = useSourcesStore()
+    const calls = []
+    stubFetch((url, init) => {
+      calls.push({ url, method: init?.method, body: init?.body ? JSON.parse(init.body) : null })
+      return { status: 202, body: { download_event_id: 7, status: 'pending' } }
+    })
+    const out = await s.checkNow(5)
+    expect(calls[0].url).toBe('/api/sources/5/check')
+    expect(calls[0].method).toBe('POST')
+    expect(out.download_event_id).toBe(7)
+  })
+
+  it('checkNow exposes the 409 download_event_id via error.body', async () => {
+    const s = useSourcesStore()
+    stubFetch(() => ({ status: 409, body: { download_event_id: 12, status: 'already_running' } }))
+    try {
+      await s.checkNow(5)
+    } catch (e) {
+      expect(e.body?.download_event_id).toBe(12)
+      return
+    }
+    throw new Error('Expected checkNow to throw on 409')
+  })
 })