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/log_config.json

@@ -0,0 +1,40 @@
+{
+  "version": 1,
+  "disable_existing_loggers": false,
+  "formatters": {
+    "default": {
+      "()": "uvicorn.logging.DefaultFormatter",
+      "fmt": "%(asctime)s %(levelprefix)s [%(name)s] %(message)s"
+    },
+    "access": {
+      "()": "uvicorn.logging.AccessFormatter",
+      "fmt": "%(asctime)s %(levelprefix)s %(client_addr)s - \"%(request_line)s\" %(status_code)s"
+    },
+    "plain": {
+      "format": "%(asctime)s %(levelname)-5s [%(name)s] %(message)s"
+    }
+  },
+  "handlers": {
+    "default": {
+      "class": "logging.StreamHandler",
+      "formatter": "default",
+      "stream": "ext://sys.stdout"
+    },
+    "access": {
+      "class": "logging.StreamHandler",
+      "formatter": "access",
+      "stream": "ext://sys.stdout"
+    },
+    "plain": {
+      "class": "logging.StreamHandler",
+      "formatter": "plain",
+      "stream": "ext://sys.stdout"
+    }
+  },
+  "loggers": {
+    "uvicorn": { "handlers": ["default"], "level": "INFO", "propagate": false },
+    "uvicorn.error": { "handlers": ["default"], "level": "INFO", "propagate": false },
+    "uvicorn.access": { "handlers": ["access"], "level": "INFO", "propagate": false },
+    "subfeed": { "handlers": ["plain"], "level": "INFO", "propagate": false }
+  }
+}