Add integration tests with docker-compose

[mikkeldamsgaard]

Summary

Add end-to-end integration tests using docker-compose to test full workflows against real services.

Requirements

• tests/docker-compose.yml with Postgres and an HTTP health-check server
• Integration test file tests/integration_test.go (build tag: integration)
• Test scenarios:
  1. wait-for TCP against Postgres → succeeds when Postgres is ready
  2. wait-for HTTP against health-check server → succeeds when server returns 200
  3. wait-for against non-existent service → fails with proper exit code after timeout
  4. render template with env vars → produces correct output file
  5. fetch from HTTP server → writes response to file
  6. exec runs command → captures output and exit code
  7. 'seed' for both mysql and postgresql. (Use postgres:16-alpine and mysql:8.0 images)
     • test create database extensively
• CI integration: add an optional integration test job to .github/workflows/ci.yml

Acceptance Criteria

• tests/docker-compose.yml with service definitions
• tests/integration_test.go with all scenarios
• Tests pass locally with docker-compose up -d && go test ./tests/ -tags=integration
• CI job ( separate workflow)
• CHANGELOG.md updated