fix(logs): keep polling reads and a channel-less account out of the warnings

The reads the frontend polls (unread counts, /api/me, the download index,
sync status, subtitle progress) are logged at DEBUG: at INFO they were ~95%
of the stored log on prod. A scheduler job failure and a playlist sync
failure carry event codes, and a Google account without a YouTube channel
has an empty playlist list instead of a warning every six hours.

backend/app/sync/playlists.py

@@ -670,11 +670,19 @@ def sync_all_playlists(db: Session) -> dict:
         try:
             with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_PULL):
                 total += sync_user_playlists(db, user).get("synced", 0)
-        except YouTubeError:
+        except YouTubeError as exc:
             db.rollback()
-            log.warning("Playlist sync failed for user %s", user.id)
+            log.warning(
+                "Playlist sync failed for user %s",
+                user.id,
+                extra=logs.event("sync.playlists.failed", user_id=user.id, reason=exc.reason),
+            )
         except Exception:
             db.rollback()
-            log.exception("Playlist sync crashed for user %s", user.id)
+            log.exception(
+                "Playlist sync crashed for user %s",
+                user.id,
+                extra=logs.event("sync.playlists.crashed", user_id=user.id),
+            )
         progress.report(i, len(users), "playlist_sync")
     return {"users": len(users), "playlists_synced": total}