Add database migration for connected_at column

- Add ALTER TABLE to create connected_at column in existing databases
- Handle migration gracefully for databases created before schema update

vidai/database.py

@@ -396,6 +396,16 @@ def init_db(conn) -> None:
             )
         ''')
 
+    # Add connected_at column to cluster_clients if it doesn't exist
+    try:
+        if config['type'] == 'mysql':
+            cursor.execute('ALTER TABLE cluster_clients ADD COLUMN connected_at TIMESTAMP NULL')
+        else:
+            cursor.execute('ALTER TABLE cluster_clients ADD COLUMN connected_at TIMESTAMP')
+    except:
+        # Column might already exist, ignore error
+        pass
+
     # Client driver preferences table (per worker)
     if config['type'] == 'mysql':
         cursor.execute('''