Integration & Contract Test Plan (Testcontainers / Playwright)

You are a Lead QA Automation Engineer specializing in modern integration testing with Testcontainers, WireMock, and real database instances.

Service / Integration Under Test:
```
@RestController
@RequestMapping("/api/v1/customers")
public class CustomerController {
    private final CustomerRepository repository;
    private final EmailClient emailClient;
    
    @PostMapping
    @ResponseStatus(HttpStatus.CREATED)
    public Customer create(@RequestBody @Valid CustomerRequest req) {
        Customer c = repository.save(new Customer(req.name(), req.email()));
        emailClient.sendWelcome(c.getEmail());
        return c;
    }
}
```

External Dependencies: PostgreSQL 16 Database + Third-Party Email HTTP API
Test Framework: Spring Boot 3.3 Test + Testcontainers PostgreSQL + WireMock

Design and generate a production-ready integration test suite:
1. **Containerized Test Lifecycle**:
   - Configure ephemeral containers (e.g. PostgreSQL, Redis, Kafka via Testcontainers).
   - Ensure clean database schema migrations (Flyway/Liquibase/Prisma) run automatically on test startup.
2. **End-to-End Workflow Verification**:
   - Write integration tests executing full user journeys (HTTP Request -> Service -> DB -> Event Emission).
   - Assert database row states directly after API calls.
3. **External API Mocking (WireMock / MSW)**:
   - Stub external third-party HTTP services (Stripe, Twilio) verifying request headers and handling 5xx error scenarios.
4. **State Cleanup & Concurrency**:
   - Ensure tests can run in parallel without cross-test data pollution (transaction rollback or database truncation).

Provide the complete integration test class implementation.