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/youtube/client.py

@@ -5,6 +5,7 @@ Public reads (channels/videos/playlistItems) use the configured API key when ava
 so they don't depend on a specific user's token; subscriptions.list?mine=true always
 uses OAuth.
 """
+import logging
 from collections.abc import Iterator
 from datetime import datetime, timedelta, timezone
 
@@ -15,6 +16,8 @@ from app.config import settings
 from app.models import User
 from app.security import decrypt
 
+log = logging.getLogger("subfeed.youtube")
+
 GOOGLE_TOKEN_URL = "https://oauth2.googleapis.com/token"
 API_BASE = "https://www.googleapis.com/youtube/v3"
 
@@ -76,6 +79,7 @@ class YouTubeClient:
         tok.expiry = now + timedelta(seconds=int(data.get("expires_in", 3600)))
         self.db.add(tok)
         self.db.commit()
+        log.info("Refreshed access token for user %s", self.user.id)
         return tok.access_token
 
     # --- core request ---
@@ -89,6 +93,7 @@ class YouTubeClient:
         resp = self._http.get(f"{API_BASE}/{path}", params=p, headers=headers)
         quota.record_usage(self.db, cost)
         if resp.status_code != 200:
+            log.warning("YouTube API %s -> %s: %s", path, resp.status_code, resp.text[:200])
             raise YouTubeError(f"GET {path} -> {resp.status_code}: {resp.text[:300]}")
         return resp.json()