Implement admin operations panel

[attaboy11]

/claim #29

Summary

• Replaced the /admin placeholder with a functional admin operations dashboard: overview metrics, user filtering/actions, job moderation, dispute handling, platform controls, and append-only audit activity.
• Expanded /api/admin/* with admin-only role enforcement, paginated/filterable admin endpoints, moderation/control mutations, and audit logging backed by the existing in-memory service style.
• Added focused API coverage for admin authorization, filtering, actions, controls, and audit output.

Demo / Evidence

• Local rendered flow verified: /admin loads, Users tab filters to review accounts, Suspend action moves the user to the suspended queue, Controls toggles Maintenance Mode, Audit tab records both actions.
• Desktop and mobile screenshots were captured locally from the production Next server during QA.

Validation

• npm run test -w apps/api -> passed
• npm run build -w apps/web -> passed
• git diff --check -> passed
• Playwright smoke flow against next start -p 3000 using system Chrome -> passed

No payout address, private token, cookie value, seed phrase, API key, or hidden runtime metadata is included.