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/auth.py

@@ -1,3 +1,4 @@
+import logging
 from datetime import datetime, timezone
 
 from authlib.integrations.starlette_client import OAuth, OAuthError
@@ -14,6 +15,8 @@ from app.security import encrypt
 # (unsubscribe, playlist export). openid/email/profile give us the account identity.
 SCOPES = "openid email profile https://www.googleapis.com/auth/youtube"
 
+log = logging.getLogger("subfeed.auth")
+
 router = APIRouter(prefix="/auth", tags=["auth"])
 
 oauth = OAuth()
@@ -53,6 +56,7 @@ async def callback(request: Request, db: Session = Depends(get_db)):
 
     email = (userinfo.get("email") or "").lower()
     if not email or email not in settings.allowed_email_set:
+        log.warning("Login denied (not on invite list): %s", email or "<no email>")
         raise HTTPException(
             status_code=403, detail="This Google account is not on the invite list."
         )
@@ -81,6 +85,7 @@ async def callback(request: Request, db: Session = Depends(get_db)):
     db.commit()
 
     request.session["user_id"] = user.id
+    log.info("Login: %s (id=%s, role=%s)", email, user.id, user.role)
     return RedirectResponse(url="/")