feat: add deployment lifecycle and rollback tracking

[Xaxxoo]

Summary

Closes #374

This PR adds a guarded CI/CD deployment lifecycle API with durable status history and rollback tracking.

What changed

• Added DeploymentsModule with deployment and immutable deployment-event entities.
• Added idempotent deployment ingestion using a CI provider's externalId.
• Added validated status transitions for received, in-progress, succeeded, failed, rollback-requested, and rolled-back deployments.
• Added filtered recent deployment queries and per-deployment event history.
• Added operator/admin-protected rollback request handling.
• Added Swagger DTOs and CI integration/rollback documentation in docs/DEPLOYMENTS.md.
• Added service tests for ingestion, idempotent retries, valid transitions, invalid transitions, and missing records.

Verification

• npm test -- --runInBand src/deployments/deployments.service.spec.ts
• npm run build:tsc
• git diff --check

All checks pass.