Implement admin operations panel

[DYSfu]

/claim #29

Summary

• replace the placeholder admin page with a functional operations dashboard for metrics, users, moderation, disputes, controls, and audit review
• add server-side admin-only API protection after JWT authentication
• add paginated and filterable admin endpoints for users, flagged listings, disputes, platform controls, and audit logs
• wire user status changes, listing decisions, dispute rulings, platform toggles, notifications, and append-only audit entries through the admin service
• add focused API coverage for non-admin rejection, filtered users, user status audit entries, moderation, dispute rulings, and platform control toggles

Demo

• demos/admin-panel-demo.mp4

Validation

• npm test
• npm run build -w apps/web
• browser check of /admin search/filter behavior
• ffprobe demos/admin-panel-demo.mp4