Implement admin operations panel

[dicnunz]

/claim #29

Summary

• Added server-side admin authorization for every /api/admin/* route, plus a Next proxy.ts route guard for /admin that returns 403 unless an admin session cookie is present.
• Expanded the admin API from a single metrics stub into paginated/filterable users, moderation, disputes, platform controls, overview, and audit-log endpoints.
• Replaced the placeholder AdminPanelPage with an operations dashboard covering metrics, trust distribution, user search/filter/actions/profile drill-in, listing moderation, dispute rulings/refund queue, platform toggles, and filterable audit history.
• Added a short demo video at demos/admin-panel-demo.mp4.

Validation

• npm test -> 5 passing API tests.
• npm run build -w apps/web -> successful Next.js production build.
• git diff --check -> clean.
• Local guard proof: curl http://localhost:3000/admin -> 403; curl -H 'Cookie: ff_role=admin' http://localhost:3000/admin -> 200 and rendered Admin Panel content.
• ffprobe demos/admin-panel-demo.mp4 -> 1280x720, 9s, 270 frames.

AI Assistance Disclosure

• Agent/tool: OpenAI Codex in the local Codex app.
• Execution mode: agent performed the code changes, tests, build, local render proof, demo-video generation, commit, and PR creation.
• Shell/tool access: yes.
• Internet access: yes, for GitHub/Algora issue and PR operations.
• Private prompts/session logs: not included.