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/components/subscriptions/SubscriptionsTab.vue

@@ -532,31 +532,23 @@ async function onCheck(source) {
   }
 }
 
-// Plan #544: arm a source for backfill mode (gallery-dl walks the full
-// post history) for the next N download runs. Default 3 — enough budget
-// to finish a deep creator without re-prompting the operator across
-// timeout boundaries. The chip on the row reflects the remaining count.
+// Plan #693: toggle a run-until-done backfill. Starting walks the full post
+// history in time-boxed chunks across ticks until it reaches the bottom (the
+// row badge tracks progress / completion); stopping cancels back to tick mode.
 async function onBackfill(source) {
-  const raw = globalThis.window?.prompt(
-    `Deep scan "${source.artist_name} (${source.platform})" — walk full history for the next how many download runs? (1–10, default 3)`,
-    '3',
-  )
-  if (raw == null) return
-  const runs = parseInt(raw, 10)
-  if (!Number.isFinite(runs) || runs < 1 || runs > 10) {
-    toast({ text: 'Deep scan: runs must be 1–10', type: 'error' })
-    return
-  }
+  const running = source.backfill_state === 'running'
   try {
-    await store.setBackfill(source.id, runs, source.artist_id)
-    toast({
-      text: `Deep scan armed for ${runs} run${runs === 1 ? '' : 's'}`,
-      type: 'success',
-    })
+    if (running) {
+      await store.stopBackfill(source.id, source.artist_id)
+      toast({ text: `Backfill stopped for ${source.artist_name}`, type: 'success' })
+    } else {
+      await store.startBackfill(source.id, source.artist_id)
+      toast({ text: `Backfill started for ${source.artist_name}`, type: 'success' })
+    }
     await store.loadAll()
   } catch (e) {
     toast({
-      text: `Deep scan failed: ${e?.detail || e?.message || e}`,
+      text: `Backfill ${running ? 'stop' : 'start'} failed: ${e?.detail || e?.message || e}`,
       type: 'error',
     })
   }