feat: add config module with environment variable handling

Implements Step 1.1 config.py with Pydantic Settings:
- PORT (default 3366), DB_PATH, BASE_PATH
- AUTH_HEADER_NAME, TRUSTED_PROXY_IPS (comma-separated)
- CSRF_SECRET (required), CSRF_COOKIE_NAME
- SEED_ON_START, LOG_LEVEL (validated), METRICS_ENABLED

Includes 28 TDD tests covering defaults, overrides, and validation.
Also fixes pytest pythonpath configuration.

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

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

pyproject.toml

@@ -47,6 +47,7 @@ ignore = ["E501"]
 
 [tool.pytest.ini_options]
 testpaths = ["tests"]
+pythonpath = ["src"]
 python_files = "test_*.py"
 python_classes = "Test*"
 python_functions = "test_*"