feat(downloads): live per-file progress on running events — #709

Now that we own the walk, surface live counts on the in-flight download in the
Downloads view. ingest_core.run takes an event_id and does a TIME-THROTTLED
write (~5s, decoupled from page boundaries so it ticks steadily regardless of
how big/slow a page is) of {downloaded, skipped, errors, quarantined, posts} to
the running download_event's metadata.live (jsonb_set; short session; status
guard so a finalized event isn't clobbered). download_backends threads
event_id from ctx; the /api/downloads list surfaces `live`; ActiveDownloadsPanel
renders it beside the elapsed timer. Native (Patreon) only — gallery-dl is an
opaque subprocess; the row only shows when `live` is present. Phase 3 overwrites
metadata with run_stats on finish, dropping `live`.

Test: _write_live_progress updates a running event's metadata.live and leaves a
finalized (status != running) event alone.

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

tests/test_patreon_ingester.py

@@ -421,6 +421,34 @@ async def test_preview_page_limit_caps_walk(source_id, sync_engine, tmp_path):
     assert result["has_more"] is True
 
 
+@pytest.mark.asyncio
+async def test_write_live_progress_updates_running_event(
+    source_id, sync_engine, tmp_path, db,
+):
+    """plan #709: live counts land in the RUNNING event's metadata.live; a
+    finalized event is left alone (status guard)."""
+    from backend.app.models import DownloadEvent
+
+    ing = _ingester(sync_engine, tmp_path, _FakeClient([]), _FakeDownloader(tmp_path))
+    running = DownloadEvent(source_id=source_id, status="running")
+    done = DownloadEvent(source_id=source_id, status="ok")
+    db.add_all([running, done])
+    await db.commit()
+
+    counts = {"downloaded": 3, "skipped": 1, "errors": 0, "quarantined": 0, "posts": 4}
+    ing._write_live_progress(running.id, counts)
+    ing._write_live_progress(done.id, {"downloaded": 9})  # guarded: status != running
+
+    live = (await db.execute(
+        select(DownloadEvent.metadata_).where(DownloadEvent.id == running.id)
+    )).scalar_one()
+    assert live["live"] == counts
+    done_meta = (await db.execute(
+        select(DownloadEvent.metadata_).where(DownloadEvent.id == done.id)
+    )).scalar_one()
+    assert "live" not in (done_meta or {})
+
+
 @pytest.mark.asyncio
 async def test_still_running_reads_backfill_state(source_id, sync_engine, tmp_path, db):
     """plan #708 B4: _still_running reflects the source's `_backfill_state` — the