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/state.py

@@ -1,8 +1,12 @@
 """Global admin-controlled app state (e.g. pausing background sync)."""
+import logging
+
 from sqlalchemy.orm import Session
 
 from app.models import AppState
 
+log = logging.getLogger("subfeed.state")
+
 
 def _row(db: Session) -> AppState:
     row = db.get(AppState, 1)
@@ -22,3 +26,4 @@ def set_sync_paused(db: Session, paused: bool) -> None:
     row.sync_paused = paused
     db.add(row)
     db.commit()
+    log.info("Background sync %s", "paused" if paused else "resumed")