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/subscriptions.py

@@ -1,4 +1,5 @@
 """Import the authenticated user's YouTube subscriptions and channel metadata."""
+import logging
 from datetime import datetime, timezone
 
 from sqlalchemy import select
@@ -7,6 +8,8 @@ from sqlalchemy.orm import Session
 from app.models import Channel, Subscription, User
 from app.youtube.client import YouTubeClient, best_thumbnail
 
+log = logging.getLogger("subfeed.sync")
+
 
 def _to_int(value) -> int | None:
     try:
@@ -107,9 +110,11 @@ def import_subscriptions(db: Session, user: User) -> dict:
             detailed = len(items)
         db.commit()
 
-    return {
+    result = {
         "subscriptions": len(fetched),
         "channels_new": new_channels,
         "channels_detailed": detailed,
         "removed_stale": removed,
     }
+    log.info("Subscription import (user %s): %s", user.id, result)
+    return result