fix: Python 3.9 compatibility for union type hints

Use Optional[T] + from __future__ import annotations instead of T | None
syntax which requires Python 3.10+.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

core/queue_db.py

@@ -1,13 +1,15 @@
 """Oracle ADB connection pool and CRUD operations for knowledge_queue."""
 
+from __future__ import annotations
+
 import json
 import os
 from contextlib import contextmanager
-from typing import Generator
+from typing import Generator, Optional
 
 import oracledb
 
-_pool: oracledb.ConnectionPool | None = None
+_pool: Optional[oracledb.ConnectionPool] = None
 
 
 def _get_pool() -> oracledb.ConnectionPool: