feat: add per-endpoint audit logs for /api/audit mutations (closes #878)

[Kaycee276]

closes #878

Description

This PR resolves the GrantFox FWC26 campaign backend issue (#878) by introducing the /api/audit router. State-changing mutations on this endpoint now securely and reliably persist audit rows.

Changes

• Implemented src/routes/audit.ts providing CRUD mutations for audit configuration.
• Added auditService.record() calls to capture (actor, action, before/after) snapshots on every mutation.
• Best-effort audit logging: failing to write the audit log gracefully falls back to structured logging without crashing the user request.
• Mounted the router to /api/audit inside src/routes/index.ts.
• Implemented comprehensive Jest tests in src/routes/audit.test.ts to guarantee stability and verify >90% coverage for changed lines.

Verification

• npm run test verifies the correct behavior.
• Lint and styles adhere to codebase standards.
• Validation checks boundary input securely.

[drips-wave]

@Kaycee276 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

[greatest0fallt1me]

LGTM ✅ green CI, clean work — merging!