Expand readiness checks into dependency-aware health reporting

[grantfox-oss]

Upgrade basic health endpoints into a dependency-aware readiness contract that checks database, Redis, queue, and service-level dependencies in a single response.

• Affected files: app/backend/src/health/health.controller.ts, app/backend/src/health/health.service.ts, app/backend/src/health/health-response.dto.ts, app/backend/src/health/public-status.dto.ts, app/backend/src/app.module.ts, app/backend/src/main.ts, app/backend/src/config/env.schema.ts, app/backend/src/metrics/metrics.service.ts, app/backend/README.md
• Acceptance criteria:
  • Readiness endpoints report service dependency health and clear failure reasons.
  • Operators can assess overall service readiness from one contract without cross-checking multiple endpoints.
  • Health output is covered by automated tests and documented for deployment operators.

[kike-alt]

Can i be assigned to work on this issue ?

[grantfox-oss]

🦊 GrantFox — @kike-alt has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #542)
2. Your PR will be reviewed by the BlockDash-Studios maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Third Campaign campaign!

@kike-alt's PR #546 was approved and merged by @MaryammAli.

🏆 @kike-alt: You earned 40 FoxPoints for this contribution! Your current tier: Builder (1,345 total points). Track your full progress on GrantFox.

👏 Great work, @kike-alt! Keep contributing to BlockDash-Studios.