feat: add centralized audit log service for sensitive admin and contract operations

[gospeltout]

Summary

• Implemented centralized audit log service (AuditLog) to record sensitive admin and contract operations (actorWallet, actionType, targetId, payload, createdAt).
• Added GET /admin/audit-log endpoint protected with adminGuard authentication.
• Added query parameter filtering by actionType and date range (fromDate, toDate, from, to, startDate, endDate).
• Implemented cursor-based pagination (default 50, capped at 100) sorted by createdAt descending.
• Enforced record immutability by exposing only the GET route with no update or delete endpoints.
• Updated bigint-serializer.utils.ts to properly serialize Date objects in API responses and updated jest.setup.ts to support dynamic Prisma model mocking.

Testing

• pnpm lint
• pnpm build
• pnpm exec prisma generate when schema or generated types changed
• Ran integration test suites:
  • pnpm test src/modules/admin/audit-log.integration.test.ts
  • pnpm test src/modules/admin/audit-log-endpoint.integration.test.ts

Checklist

• Linked issue or backlog item
• No secrets or live credentials added
• Docs updated if setup or env changed
• Change is scoped to one problem

Closes #898