Revert database placeholders back to ? for SQLite compatibility

vidai/database.py

@@ -755,7 +755,7 @@ def get_user_by_id(user_id: int) -> Optional[Dict[str, Any]]:
     """Get user by ID."""
     conn = get_db_connection()
     cursor = conn.cursor()
-    cursor.execute('SELECT id, username, email, role, active, created_at, last_login, avatar FROM users WHERE id = %s',
+    cursor.execute('SELECT id, username, email, role, active, created_at, last_login, avatar FROM users WHERE id = ?',
                    (user_id,))
     row = cursor.fetchone()
     conn.close()
@@ -1795,7 +1795,7 @@ def update_user_avatar(user_id: int, avatar_filename: str = None) -> bool:
     """Update user avatar filename."""
     conn = get_db_connection()
     cursor = conn.cursor()
-    cursor.execute('UPDATE users SET avatar = %s WHERE id = %s', (avatar_filename, user_id))
+    cursor.execute('UPDATE users SET avatar = ? WHERE id = ?', (avatar_filename, user_id))
     conn.commit()
     success = cursor.rowcount > 0
     conn.close()