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/entrypoint.sh

@@ -5,4 +5,4 @@ echo "Applying database migrations..."
 alembic upgrade head
 
 echo "Starting Subfeed API..."
-exec uvicorn app.main:app --host 0.0.0.0 --port 8000
+exec uvicorn app.main:app --host 0.0.0.0 --port 8000 --log-config log_config.json