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/main.py

@@ -5,15 +5,20 @@ from pathlib import Path
 
 from fastapi import FastAPI, HTTPException
 
-# Make our own loggers (e.g. the scheduler) visible in container logs — uvicorn's
-# logging config otherwise filters out INFO from non-uvicorn loggers.
+# When started via uvicorn --log-config the "subfeed" logger is already configured
+# (see log_config.json). This block is a timestamped fallback for other entrypoints
+# (tests, scripts) so our logs are never silently dropped.
 _subfeed_logger = logging.getLogger("subfeed")
 if not _subfeed_logger.handlers:
     _handler = logging.StreamHandler(sys.stdout)
-    _handler.setFormatter(logging.Formatter("%(levelname)s [%(name)s] %(message)s"))
+    _handler.setFormatter(
+        logging.Formatter("%(asctime)s %(levelname)-5s [%(name)s] %(message)s")
+    )
     _subfeed_logger.addHandler(_handler)
-_subfeed_logger.setLevel(logging.INFO)
-_subfeed_logger.propagate = False
+    _subfeed_logger.setLevel(logging.INFO)
+    _subfeed_logger.propagate = False
+
+log = logging.getLogger("subfeed.app")
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import FileResponse
 from fastapi.staticfiles import StaticFiles
@@ -27,10 +32,12 @@ from app.scheduler import shutdown_scheduler, start_scheduler
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
+    log.info("Subfeed starting up")
     start_scheduler()
     try:
         yield
     finally:
+        log.info("Subfeed shutting down")
         shutdown_scheduler()