feat(scheduler): report progress from every job + parallelize RSS polling

Every background job now reports progress, so the dashboard can show a live
bar for any run (not only enrich/backfill/shorts/maintenance): rss_poll and
subscriptions (runner), autotag, and playlist_sync gained progress.report
calls over their loops.

RSS polling now fetches the channel feeds concurrently (16 workers) instead
of one-at-a-time — a slow/unreachable feed no longer blocks the rest, cutting
a full poll of the catalogue from minutes to seconds. Feed fetches are
network-bound and run in the pool; DB inserts stay on the session's thread
(SQLAlchemy sessions aren't thread-safe), applied as each fetch completes.
Split the DB-write half of poll_rss_channel into apply_rss_feed so the fetch
and apply phases compose cleanly.

backend/app/sync/playlists.py

@@ -12,7 +12,7 @@ from datetime import datetime, timezone
 from sqlalchemy import delete, select
 from sqlalchemy.orm import Session
 
-from app import quota
+from app import progress, quota
 from app.auth import has_read_scope, has_write_scope
 from app.models import Channel, OAuthToken, Playlist, PlaylistItem, User, Video
 from app.sync.videos import apply_video_details, parse_dt
@@ -382,8 +382,9 @@ def sync_all_playlists(db: Session) -> dict:
         .all()
     )
     total = 0
-    for user in users:
+    for i, user in enumerate(users, 1):
         if not has_read_scope(user):
+            progress.report(i, len(users), "playlist_sync")
             continue
         try:
             with quota.attribute(user.id, "playlist_sync"):
@@ -394,4 +395,5 @@ def sync_all_playlists(db: Session) -> dict:
         except Exception:
             db.rollback()
             log.exception("Playlist sync crashed for user %s", user.id)
+        progress.report(i, len(users), "playlist_sync")
     return {"users": len(users), "playlists_synced": total}