Add account creation route with validation. Delegates to
CardDAVSync.addAccount() which creates account and discovers
addressbooks. Returns 201 with account + addressbooks array.
Add _mockTestConnection() helper for testing CardDAV routes without
real server connections.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
Add CardDAV API router with GET /accounts endpoint. Returns all
CardDAV accounts from database via cardav-sync service. Added test
infrastructure with _setTestDatabase helper for isolated API testing.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>