test: Add unit tests to reach 50% code coverage

Adds 89 new tests bringing total from 12 to 101. Coverage went
from 44% to 52%.

New test coverage:
- AuthService: login flow, account lockout, token refresh
- DocumentService: pagination, delete, batch generation
- TemplateService: full CRUD operations
- ExceptionHandlingMiddleware: all exception type mappings
- Validators: RegisterDto, LoginDto, CreateTemplateDto, etc.
- PaginatedResult: edge cases for pagination logic
- AuthController & ControllerExtensions: basic unit tests

Also adds coverage/ to .gitignore since test runs generate
reports there.