Add database-backed audit logs for mutations

[zhangsiqiang519]

Summary

• add an AuditLog entity, migration, module, and AuditService
• persist audit events for bounty create/update/delete/restore
• persist audit events for submission create/approve/reject
• scrub sensitive metadata keys before saving audit records
• pass authenticated wallet address into bounty mutations for actor attribution
• add focused unit coverage and changelog entry

Closes #198

Verification

• git diff --check
• npm run build --workspace=apps/backend
• npm run lint --workspace=apps/backend
• npm run test --workspace=apps/backend -- audit.service.spec.ts bounties.service.spec.ts submissions.service.spec.ts submissions.contract-errors.spec.ts

[GBOYEE]

I'd like to work on this issue. I have background in backend and will submit a PR ASAP.

[GBOYEE]

I'd like to work on this.

Approach:

• I'll add the audit logging with proper schema and query support
• Verify with existing tests + add new ones if needed

Estimated effort: ~1-2 hours. PR incoming shortly.
Closes #298

[Xuccessor]

Thanks for this. Issue #198 is assigned to @barry01-hash — only the assigned contributor's PR is being merged here, so I'll hold off. Worth checking in on the issue.

[Xuccessor]

Closing this — #198 is assigned to @barry01-hash. Thanks for putting this together!