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>

backend/app/api/downloads.py

@@ -44,6 +44,9 @@ def _list_record(event: DownloadEvent, source: Source | None, artist: Artist | N
         "bytes_downloaded": event.bytes_downloaded,
         "error": event.error,
         "summary": _summary_from_metadata(event.metadata_),
+        # plan #709: mid-walk live counts for a RUNNING native-ingester event
+        # (None otherwise; phase 3 overwrites metadata with run_stats on finish).
+        "live": (event.metadata_ or {}).get("live"),
     }