Bugfix pagination not handled

[stianjsu]

Fixes pagination handling across all CRUD endpoints. The @bp.paginate() decorator silently enforced a default page size of 10, which meant newly created events (and other entities) wouldn't show up in the frontend unless they fell within the first page. Removed the decorator entirely in favor of direct service calls with optional pagination parameters.

Key additions

• Removed @bp.paginate() decorator from events, groups, restaurants, and slack_users CRUD endpoints — the decorator wasn't compatible with how responses were being returned
• Made page and per_page optional (defaulting to None) in EventService, GroupService, and RestaurantService so endpoints work without pagination parameters
• Added default ordering by event_time descending in EventRepository so events come back in a sensible order without relying on the paginator
• Added TESTING.md documenting current test coverage, known gaps, and recommendations
• Updated Dockerfile.backend with PostgreSQL client tools and added run-tests.sh for running tests inside Docker

How Has This Been Tested?

• Updated service tests (test_event_service, test_group_service, test_restaurant_service, test_slack_user_service) to match the new optional pagination signatures