feat(be): add release pipeline for backend (BE-30)

[NnamdiCyber]

Summary

Implements the full automated release pipeline for the backend (#416 BE-30).

Changes

• backend-ci.yml — lint → type-check → build → unit + integration tests on every push/PR
• backend-release.yml — on push to main: build Docker image → migrate → deploy staging → deploy production (manual approval gate); manual rollback job via workflow_dispatch
• actions/run-migrations — composite action: runs supabase db push, counts applied migrations, emits sha/env/timestamp annotation
• actions/deploy-app — composite action: triggers deploy, polls /health up to 4 min, emits release metadata annotation
• Dockerfile + .dockerignore — multi-stage build for the NestJS backend

Acceptance Criteria

• ✅ Repeatable, automated releases from main
• ✅ Migrations applied exactly once per deploy; count + metadata logged as GitHub Actions annotations
• ✅ Rollbacks re-deploy a previous image by SHA (no reverse migrations — forward-only is the safe default)
• ✅ Release metadata logged: git sha, image tag, environment, timestamp

Notes

• The deploy-app action's deploy trigger is a placeholder curl — replace with the actual platform webhook (Render/Railway/Fly.io)
• Production requires a manual reviewer approval gate (Settings → Environments → production)

Closes #416

[drips-wave]

@NnamdiCyber Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[Cedarich]

Fix worflow

[Cedarich]

Fix failing workflow

[Cedarich]

Fix workflow

[Cedarich]

Kindly remove changes made to the backend.yml file