[infra] Add gateway and API services to docker-compose.yml

[yinkscss]

Context

Local development currently starts only Postgres via docker-compose.yml. The PRD and README describe two Rust services (gateway and API) that developers must run manually with cargo run.

Problem

Contributors lack a one-command local stack mirroring production topology. This slows onboarding and makes webhook testing harder to reproduce consistently.

Proposed scope

• Add waveflow-gateway and waveflow-api services to docker-compose.yml
• Wire DATABASE_URL, ports 8080/8081, and env file passthrough from .env
• Depend on healthy Postgres before starting app containers
• Document the compose workflow in README quick start

Acceptance criteria

• docker-compose up starts Postgres, gateway, and API
• Gateway health check responds at http://localhost:8080/health
• API health check responds at http://localhost:8081/health
• Migrations run automatically on service start (existing sqlx behavior)
• README documents compose-based dev flow

References

• docker-compose.yml
• Dockerfile.gateway, Dockerfile.api
• README.md
• docs/ROADMAP.md (Phase 0 scaffold)

[BojayL]

/claim

Opened PR #50 with gateway/API compose services and local stack docs.