Fix: user_id 'admin' rejected by Jellyfin API (400)

The Jellyfin service config's user_id field accepts either the internal
Jellyfin user ID (a long hash) or a username (e.g. 'admin'). The worker
passed the raw value directly to client.libraries(user_id), but Jellyfin's
API rejects usernames with a 400.

Now validates the configured user_id against the Jellyfin users API:
1. If it matches a user's Id (internal hash), use it directly.
2. If it matches a user's Name (username like 'admin'), resolve the Id.
3. If no match, fall back to the first user and log a warning.

283 backend tests pass; ruff clean.

backend/src/media_library_viewer_api/workers/media_index_worker.py

@@ -113,6 +113,29 @@ def _resolve_jellyfin(service_id: str) -> tuple[Any, str]:
         if not users:
             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:
+            match = next((u for u in users if str(u.get("Name", "")) == user_id), None)
+            if match:
+                user_id = match["Id"]
+                logger.info(
+                    "Resolved username '%s' to Jellyfin Id '%s'",
+                    service.get("config", {}).get("user_id"),
+                    user_id,
+                )
+            elif users:
+                user_id = users[0]["Id"]
+                logger.warning(
+                    "user_id '%s' not found; falling back to first user '%s'",
+                    service.get("config", {}).get("user_id"),
+                    user_id,
+                )
     return client, user_id