chore: add ruff linter, fix mypy errors, and write README

- Add ruff linter configuration to pyproject.toml with modern Python 3.12 rules
- Add ruff~=0.9.4 to dev dependencies
- Fix all mypy type errors (Optional[] hints, Token types, Any returns)
- Auto-fix 54 ruff issues (import sorting, Optional -> X | None syntax)
- Create ProjectMeta dataclass for single source of truth from pyproject.toml
- Write comprehensive README.md with setup, config, and development docs
- Update main.py to use settings.app_description from pyproject.toml

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/conftest.py

@@ -2,7 +2,7 @@
 Pytest configuration and fixtures.
 """
 import pytest
-from httpx import AsyncClient, ASGITransport
+from httpx import ASGITransport, AsyncClient
 
 from src.main import app