feat(subscriptions): live posts-processed progress on backfill/recovery — #704 step 2

The running badge only showed the chunk counter; now it shows posts walked
— real walk progress. The ingester already reports posts_processed per
chunk (step 1); the backfill lifecycle accumulates it into
config_overrides._backfill_posts across chunks. SourceRecord exposes
backfill_posts; start_backfill/start_recovery clear it (fresh walk); stop
clears it too. SourceRow/SourceCard badge renders "Recovering · 45 posts"
(falls back to "(N)" chunks before any posts are counted).

Per-chunk accumulation (no mid-walk DB write) — simple and race-free; a
small over-count from each chunk re-walking its resumed page is fine for a
progress indicator.

Tests: lifecycle accumulates posts_processed across chunks; start clears a
prior _backfill_posts and the record exposes it.

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

frontend/src/components/subscriptions/SourceCard.vue

@@ -28,7 +28,9 @@
         v-else-if="source.backfill_state === 'running'"
         size="x-small" color="info" variant="tonal" label
       >{{ source.backfill_bypass_seen ? 'Recovering' : 'Backfilling'
-        }}{{ source.backfill_chunks ? ` (${source.backfill_chunks})` : '' }}</v-chip>
+        }}{{ source.backfill_posts
+              ? ` · ${source.backfill_posts} posts`
+              : (source.backfill_chunks ? ` (${source.backfill_chunks})` : '') }}</v-chip>
       <v-chip
         v-else-if="source.backfill_state === 'complete'"
         size="x-small" color="success" variant="tonal" label

frontend/src/components/subscriptions/SourceRow.vue

@@ -35,7 +35,9 @@
         v-else-if="source.backfill_state === 'running'"
         size="x-small" color="info" variant="tonal" label
       >{{ source.backfill_bypass_seen ? 'Recovering' : 'Backfilling'
-        }}{{ source.backfill_chunks ? ` (${source.backfill_chunks})` : '' }}</v-chip>
+        }}{{ source.backfill_posts
+              ? ` · ${source.backfill_posts} posts`
+              : (source.backfill_chunks ? ` (${source.backfill_chunks})` : '') }}</v-chip>
       <v-chip
         v-else-if="source.backfill_state === 'complete'"
         size="x-small" color="success" variant="tonal" label