docs: add comprehensive API reference documentation

[blvckmamba81-sketch]

Summary

Adds a comprehensive API reference document at docs/API_REFERENCE.md covering every endpoint in the StarkEd backend API.

What's included

• 16 API modules fully documented under /api/v1/
  • Health (liveness, readiness, full status)
  • Auth (register, login, profile, role management, user admin)
  • Content / IPFS (upload, batch upload, retrieve, pin/unpin, cache, node info)
  • Courses & Versions (create/list/compare/restore/export versions, settings, statistics)
  • Enrollments (full CRUD, progress, waitlist, certificates, analytics, bulk ops)
  • Payments (Stellar payments, intents, refunds, webhooks, exchange rates)
  • Quizzes (CRUD, publish, submit, grading, statistics)
  • Assignments (create, submit, grade, bulk grade, stats)
  • Analytics (overview, reports, enrollment trends, completion rates, export)
  • Search (full-text, voice, suggestions, recommendations, history, alerts)
  • Gamification (leaderboard, achievements, events, points, challenges)
  • Notifications (history, read/unread, preferences)
  • Time-Lock Credentials (issue, release, batch release, emergency revoke, audit)
  • VRF — Verifiable Random Function (request, generate, beacon, commit-reveal)
  • Holographic Storage (encode, decode, parallel access, metrics, optimize)

Documentation standards met

• ✅ All REST endpoints documented with HTTP method, path, and description
• ✅ Authentication requirements specified per endpoint
• ✅ Rate limit tiers documented with limits and windows
• ✅ Request body fields with types, required flags, and descriptions
• ✅ Query/path parameters documented
• ✅ Code examples in both cURL and JavaScript for key endpoints
• ✅ Request and response JSON examples
• ✅ Error codes and response shapes documented
• ✅ Consistent formatting throughout

Testing

Documentation-only change — no runtime code modified.

Closes #204