feat(subscriptions): smarter-backfill UI — Start/Stop + state badge

Plan #693 (frontend). Backend landed in 96fffaf.

- sources store: setBackfill(runs) → startBackfill/stopBackfill ({action}).
- SubscriptionsTab: the deep-scan window.prompt for N runs becomes a
  Start/Stop toggle keyed on source.backfill_state.
- SourceRow + SourceCard: the 'backfill (N×)' chip becomes a state badge —
  Backfilling (chunk N) / Backfilled / Stalled — and the scan button
  toggles between Backfill (mdi-magnify-scan) and Stop (mdi-stop).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/stores/sources.js

@@ -85,11 +85,16 @@ export const useSourcesStore = defineStore('sources', () => {
     }
   }
 
-  // Plan #544: arm a source for backfill mode. The next `runs` download
-  // runs (default 3) walk gallery-dl's full post history instead of
-  // exiting early at the first contiguous archived block.
-  async function setBackfill(id, runs = 3, artistIdHint = null) {
-    const body = await api.post(`/api/sources/${id}/backfill`, { body: { runs } })
+  // Plan #693: start/stop a run-until-done backfill. 'start' walks the full
+  // post history in time-boxed chunks until it reaches the bottom (then the
+  // source shows backfill_state 'complete'); 'stop' cancels back to tick mode.
+  async function startBackfill(id, artistIdHint = null) {
+    const body = await api.post(`/api/sources/${id}/backfill`, { body: { action: 'start' } })
+    _invalidate(artistIdHint ?? body.artist_id)
+    return body
+  }
+  async function stopBackfill(id, artistIdHint = null) {
+    const body = await api.post(`/api/sources/${id}/backfill`, { body: { action: 'stop' } })
     _invalidate(artistIdHint ?? body.artist_id)
     return body
   }
@@ -120,7 +125,8 @@ export const useSourcesStore = defineStore('sources', () => {
     loadAll, loadForArtist,
     create, update, remove,
     checkNow,
-    setBackfill,
+    startBackfill,
+    stopBackfill,
     findOrCreateArtist, autocompleteArtist,
     loadScheduleStatus,
     sourcesByArtistGrouped,