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/SourceRow.vue

@@ -32,11 +32,17 @@
         size="x-small" color="error" variant="tonal" label
       >{{ source.consecutive_failures }}</v-chip>
       <v-chip
-        v-else-if="(source.backfill_runs_remaining || 0) > 0"
+        v-else-if="source.backfill_state === 'running'"
         size="x-small" color="info" variant="tonal" label
-      >
-        backfill ({{ source.backfill_runs_remaining }}×)
-      </v-chip>
+      >Backfilling{{ source.backfill_chunks ? ` (${source.backfill_chunks})` : '' }}</v-chip>
+      <v-chip
+        v-else-if="source.backfill_state === 'complete'"
+        size="x-small" color="success" variant="tonal" label
+      >Backfilled</v-chip>
+      <v-chip
+        v-else-if="source.backfill_state === 'stalled'"
+        size="x-small" color="warning" variant="tonal" label
+      >Stalled</v-chip>
       <span v-else class="fc-source-row__zero">0</span>
     </td>
     <td class="fc-source-row__actions">
@@ -49,13 +55,15 @@
         <v-tooltip activator="parent" location="top">Check now</v-tooltip>
       </v-btn>
       <v-btn
-        icon="mdi-magnify-scan" size="x-small" variant="text"
-        :disabled="(source.backfill_runs_remaining || 0) > 0"
+        size="x-small" variant="text"
+        :color="source.backfill_state === 'running' ? 'warning' : undefined"
         @click.stop="$emit('backfill', source)"
       >
-        <v-icon>mdi-magnify-scan</v-icon>
+        <v-icon>{{ source.backfill_state === 'running' ? 'mdi-stop' : 'mdi-magnify-scan' }}</v-icon>
         <v-tooltip activator="parent" location="top">
-          Deep scan — walk full history for next few runs
+          {{ source.backfill_state === 'running'
+            ? 'Stop backfill'
+            : 'Backfill — walk the full post history until complete' }}
         </v-tooltip>
       </v-btn>
       <v-btn