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/SourceCard.vue

@@ -25,9 +25,17 @@
         size="x-small" color="error" variant="tonal" label
       >{{ source.consecutive_failures }} err</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>
     </div>
 
     <div class="fc-source-card__actions">
@@ -40,11 +48,13 @@
       </v-btn>
       <v-btn
         size="x-small" variant="text"
-        :disabled="(source.backfill_runs_remaining || 0) > 0"
+        :color="source.backfill_state === 'running' ? 'warning' : undefined"
         @click.stop="$emit('backfill', source)"
       >
-        <v-icon>mdi-magnify-scan</v-icon>
-        <v-tooltip activator="parent" location="top">Deep scan</v-tooltip>
+        <v-icon>{{ source.backfill_state === 'running' ? 'mdi-stop' : 'mdi-magnify-scan' }}</v-icon>
+        <v-tooltip activator="parent" location="top">
+          {{ source.backfill_state === 'running' ? 'Stop backfill' : 'Backfill full history' }}
+        </v-tooltip>
       </v-btn>
       <v-btn size="x-small" variant="text" @click.stop="$emit('edit', source)">
         <v-icon>mdi-pencil</v-icon>