feat: Add comprehensive test suite with 46 passing tests

- Add pytest configuration in pyproject.toml with markers and async support
- Create shared test fixtures in tests/conftest.py for config, auth, and sample data
- Implement unit tests for all major components:
  * Configuration management (11 tests) - TOML loading/saving, singleton pattern
  * FastAPI API endpoints (12 tests) - Banks, accounts, transactions with mocks
  * CLI API client (11 tests) - HTTP client integration and error handling
  * Background scheduler (12 tests) - APScheduler job management and async ops

- Fix GoCardless API authentication mocking by adding token endpoints
- Resolve TOML file writing issues (binary vs text mode for tomli_w)
- Add comprehensive testing documentation to README
- Update code structure documentation to include test organization

Testing framework includes:
- respx for HTTP request mocking
- pytest-asyncio for async test support
- pytest-mock for advanced mocking capabilities
- requests-mock for CLI HTTP client testing
- Realistic test data fixtures for banks, accounts, and transactions

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

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

README.md

@@ -260,6 +260,38 @@ uv run leggend --reload
 uv run leggen status
 ```
 
+### Testing
+
+Run the comprehensive test suite with:
+
+```bash
+# Run all tests
+uv run pytest
+
+# Run unit tests only  
+uv run pytest tests/unit/
+
+# Run with verbose output
+uv run pytest tests/unit/ -v
+
+# Run specific test files
+uv run pytest tests/unit/test_config.py -v
+uv run pytest tests/unit/test_scheduler.py -v
+uv run pytest tests/unit/test_api_banks.py -v
+
+# Run tests by markers
+uv run pytest -m unit      # Unit tests
+uv run pytest -m api       # API endpoint tests
+uv run pytest -m cli       # CLI tests
+```
+
+The test suite includes:
+- **Configuration management tests** - TOML config loading/saving
+- **API endpoint tests** - FastAPI route testing with mocked dependencies
+- **CLI API client tests** - HTTP client integration testing
+- **Background scheduler tests** - APScheduler job management
+- **Mock data and fixtures** - Realistic test data for banks, accounts, transactions
+
 ### Code Structure
 ```
 leggen/              # CLI application
@@ -272,6 +304,15 @@ leggend/             # FastAPI backend service
 ├── services/        # Business logic
 ├── background/      # Background job scheduler
 └── main.py          # FastAPI application
+
+tests/               # Test suite
+├── conftest.py      # Shared test fixtures
+└── unit/            # Unit tests
+    ├── test_config.py      # Configuration tests
+    ├── test_scheduler.py   # Background scheduler tests
+    ├── test_api_banks.py   # Banks API tests
+    ├── test_api_accounts.py # Accounts API tests
+    └── test_api_client.py  # CLI API client tests
 ```
 
 ### Contributing