chore: structured timestamped logging across the backend

- uvicorn --log-config (log_config.json): timestamped formatters for uvicorn
  access/error and the app's own "subfeed" loggers (ms precision)
- Log key activity: startup/shutdown, login/denied, token refresh, YouTube API
  errors, subscription imports, sync pause/resume
- Background sync jobs no longer swallow errors silently — failures in RSS poll,
  backfill, enrichment, autotag and resync are logged with tracebacks

backend/app/sync/runner.py

@@ -4,11 +4,15 @@ Channel/video data is shared across users, so background work acts through a "se
 user" (any user with a stored refresh token) unless a YOUTUBE_API_KEY is configured for
 public reads.
 """
+import logging
+
 from sqlalchemy import select
 from sqlalchemy.orm import Session
 
 from app import quota
 from app.config import settings
+
+log = logging.getLogger("subfeed.sync")
 from app.models import Channel, OAuthToken, User
 from app.sync.subscriptions import import_subscriptions
 from app.sync.videos import (
@@ -43,6 +47,7 @@ def run_rss_poll(db: Session, channels: list[Channel] | None = None) -> int:
             new += poll_rss_channel(db, channel)
         except Exception:
             db.rollback()
+            log.exception("RSS poll failed for channel %s", channel.id)
     return new
 
 
@@ -83,6 +88,7 @@ def run_subscription_resync(db: Session) -> dict:
             import_subscriptions(db, user)
         except Exception:
             db.rollback()
+            log.exception("Subscription resync failed for user %s", user.id)
     return {"users": len(users)}
 
 
@@ -111,6 +117,7 @@ def run_recent_backfill(
                 processed += 1
             except Exception:
                 db.rollback()
+                log.exception("Recent backfill failed for channel %s", channel.id)
     return {"channels_processed": processed, "videos_added": videos_added}
 
 
@@ -139,4 +146,5 @@ def run_deep_backfill(db: Session, max_channels: int = 10, max_pages: int = 5) -
                 processed += 1
             except Exception:
                 db.rollback()
+                log.exception("Deep backfill failed for channel %s", channel.id)
     return {"channels_processed": processed, "videos_added": videos_added}