Fix environment variable sync when .env loaded after import

- Move env sync logic into reusable function
- Patch ServiceClient and AsyncTinker constructors to re-sync env
- Ensures load_dotenv() works correctly even after importing mint

Improve environment variable handling and URL scheme support

- Add mint:// URL scheme support for training endpoints
- Prefer MINT_* environment variables when both MINT and TINKER are set
- Normalize legacy TINKER_APIKEY to TINKER_API_KEY format
- Default to mint.macaron.im when MINT_API_KEY is set without base URL
- Update documentation to clarify environment variable precedence