feat: add model management endpoints and admin interface for cache status

Co-authored-by: Copilot <copilot@github.com>

backend/app/services/models.py

@@ -198,3 +198,12 @@ def get_model_output_modalities(
         return json.loads(row[0])
     except (json.JSONDecodeError, TypeError):
         return []
+
+
+def get_cache_status(conn: duckdb.DuckDBPyConnection) -> dict[str, Any]:
+    """Return cache last update time and model count."""
+    row = conn.execute(
+        "SELECT MAX(fetched_at), COUNT(*) FROM models_cache"
+    ).fetchone()
+    last_updated, model_count = (row[0], row[1]) if row else (None, 0)
+    return {"last_updated": last_updated, "model_count": model_count}