Create Comprehensive Integration Test Suite

## Overview
Build a complete integration test suite that tests API endpoints with a real database and validates end-to-end flows.

## Business Value
Integration tests catch issues that unit tests miss, ensuring the system works correctly as a whole.

## Current State
- No integration tests
- Only unit tests exist
- End-to-end flows untested

## Subtasks
- [ ] Set up test database
- [ ] Create test fixtures
- [ ] Write API endpoint tests
- [ ] Test authentication flows
- [ ] Test CRUD operations
- [ ] Test error handling
- [ ] Add test cleanup
- [ ] Integrate with CI

## Implementation Steps
1. Set up test database container
2. Create test helpers
3. Write tests for each endpoint
4. Add test data fixtures
5. Integrate with CI pipeline

## Acceptance Criteria
- [ ] All API endpoints have integration tests
- [ ] Tests use real database
- [ ] Tests are isolated and independent
- [ ] Tests run in CI
- [ ] Test execution is fast

## Estimated Effort
- Hours: 24-32
- Complexity: Medium