Fix: user_id validation causes read timeout on slow Jellyfin

The previous fix called client.users() unconditionally to validate the
user_id, adding an extra HTTP round-trip before the build. On a slow
Jellyfin connection this burned through the 10s timeout before the
actual libraries() call.

Restructured to try libraries(user_id) directly first. Only when that
fails does the worker resolve the username via the users API. So:
- Correctly configured user_id (internal hash): zero extra round-trips.
- Username like 'admin': libraries() fails → users() resolves it → retry.

283 backend tests pass; ruff clean.

backend/src/media_library_viewer_api/workers/media_index_worker.py

@@ -114,27 +114,29 @@ def _resolve_jellyfin(service_id: str) -> tuple[Any, str]:
             raise RuntimeError("No Jellyfin users found and no user_id configured on the service")
         user_id = users[0]["Id"]
     else:
-        # The config field accepts either a Jellyfin internal user ID (a long
-        # hash) or a username (e.g. "admin"). Validate against the users API:
-        # if the configured value doesn't match any user's Id, try matching by
-        # Name, then fall back to the first user.
-        users = client.users()
-        valid_ids = {str(u.get("Id", "")) for u in users}
-        if user_id not in valid_ids:
+        # Try the configured user_id directly. It might be the internal
+        # Jellyfin Id (a long hash) — in that case libraries() succeeds
+        # without an extra users() round-trip. Only if it fails do we
+        # resolve it via the users API (the config field accepts usernames
+        # like 'admin' too, but Jellyfin's API rejects them on /Users/<id>).
+        try:
+            client.libraries(user_id)
+        except Exception:
+            users = client.users()
             match = next((u for u in users if str(u.get("Name", "")) == user_id), None)
             if match:
-                user_id = match["Id"]
+                resolved = match["Id"]
                 logger.info(
                     "Resolved username '%s' to Jellyfin Id '%s'",
-                    service.get("config", {}).get("user_id"),
                     user_id,
+                    resolved,
                 )
+                user_id = resolved
             elif users:
                 user_id = users[0]["Id"]
                 logger.warning(
-                    "user_id '%s' not found; falling back to first user '%s'",
+                    "user_id '%s' not found; falling back to first user",
                     service.get("config", {}).get("user_id"),
-                    user_id,
                 )
     return client, user_id