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>

.claude/settings.local.json

@@ -1,7 +1,9 @@
 {
   "permissions": {
     "allow": [
-      "Bash(mkdir:*)"
+      "Bash(mkdir:*)",
+      "Bash(uv sync:*)",
+      "Bash(uv run pytest:*)"
     ],
     "deny": [],
     "ask": []