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>

pyproject.toml

@@ -88,7 +88,7 @@ leann-backend-diskann = { path = "packages/leann-backend-diskann", editable = tr
 leann-backend-hnsw = { path = "packages/leann-backend-hnsw", editable = true }
 
 [tool.ruff]
-target-version = "py310"
+target-version = "py39"
 line-length = 100
 extend-exclude = [
     "third_party",