fix: clean build system and Python 3.9 compatibility

Build system improvements:
- Simplify macOS environment detection using brew --prefix
- Remove complex hardcoded paths and CMAKE_ARGS
- Let CMake automatically find Homebrew packages via CMAKE_PREFIX_PATH
- Clean separation between Intel (/usr/local) and Apple Silicon (/opt/homebrew)

Python 3.9 compatibility:
- Set ruff target-version to py39 to match project requirements
- Replace str | None with Union[str, None] in type annotations
- Add Union imports where needed
- Fix core interface, CLI, chat, and embedding server files

🤖 Generated with [Claude Code](https://claude.ai/code)

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

packages/leann-backend-hnsw/leann_backend_hnsw/hnsw_embedding_server.py

@@ -10,6 +10,7 @@ import sys
 import threading
 import time
 from pathlib import Path
+from typing import Union
 
 import msgpack
 import numpy as np
@@ -33,7 +34,7 @@ if not logger.handlers:
 
 
 def create_hnsw_embedding_server(
-    passages_file: str | None = None,
+    passages_file: Union[str, None] = None,
     zmq_port: int = 5555,
     model_name: str = "sentence-transformers/all-mpnet-base-v2",
     distance_metric: str = "mips",