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/vector.py

@@ -1,9 +1,11 @@
 """Embedding generation and Oracle vector store insertion."""
 
+from __future__ import annotations
+
 import array
 import os
 from contextlib import contextmanager
-from typing import Generator
+from typing import Generator, Optional
 
 import oci
 import oracledb
@@ -14,7 +16,7 @@ from oci.generative_ai_inference.models import (
 )
 
 # Reuse same pool as queue_db but connect to same ADB instance
-_pool: oracledb.ConnectionPool | None = None
+_pool: Optional[oracledb.ConnectionPool] = None
 
 
 def _get_pool() -> oracledb.ConnectionPool: