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>

tests/test_source_service.py

@@ -238,15 +238,25 @@ async def test_start_recovery_arms_bypass_flag(db):
         artist_id=artist.id, platform="patreon",
         url="https://patreon.com/alice",
     )
+    # Simulate a prior walk's posts counter — start must clear it (plan #704).
+    await db.execute(
+        Source.__table__.update().where(Source.id == rec.id).values(
+            config_overrides={"_backfill_posts": 42},
+        )
+    )
+    await db.commit()
+
     updated = await svc.start_recovery(rec.id)
     assert updated.backfill_state == "running"
     assert updated.backfill_bypass_seen is True
+    assert updated.backfill_posts == 0  # cleared for a fresh walk
     assert updated.backfill_runs_remaining == BACKFILL_MAX_CHUNKS
 
     co = (await db.execute(
         select(Source.config_overrides).where(Source.id == rec.id)
     )).scalar_one()
     assert co.get("_backfill_bypass_seen") is True
+    assert "_backfill_posts" not in co
 
     # Stop clears the bypass flag too (shared lifecycle).
     stopped = await svc.stop_backfill(rec.id)