Standardize API error envelopes and validation semantics

[grantfox-oss]

Unify validation and business-error responses so clients receive deterministic error payloads across controllers, guards, and queue-driven failures.

• Affected files: app/backend/src/main.ts, app/backend/src/common/filters/global-http-exception.filter.ts, app/backend/src/common/utils/validation-error.mapper.ts, app/backend/src/auth/guards/custom-throttler.guard.ts, app/backend/src/metrics/metrics.interceptor.ts, app/backend/src/audit/audit.service.ts, app/backend/src/marketplace/marketplace.controller.ts, app/backend/test/health.e2e-spec.ts
• Acceptance criteria:
  • Validation errors, auth failures, rate-limit events, and domain exceptions return a consistent contract.
  • Error payloads include stable codes, human-readable messages, and trace correlation metadata.
  • Critical routes are test-covered to prevent regressions in API contract behavior.

[Dev-Zully]

Am a fullstack developer with experience in different lanaguages i have read this issue requirment and am confident to achieve it

[grantfox-oss]

🦊 GrantFox — @Dev-Zully 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 #562)
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!

@Dev-Zully's PR #564 was approved and merged by @MaryammAli.

🏆 @Dev-Zully: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (917 total points). Track your full progress on GrantFox.

👏 Great work, @Dev-Zully! Keep contributing to BlockDash-Studios.