Add OpenAPI 3.1 documentation — full Mobile API spec (59 endpoints)

Complete OpenAPI 3.1.0 YAML specification covering all mobile API endpoints:
- 15 tags (Auth, Dashboard, Participants, Notifications, Shop, Payments, etc.)
- 10 reusable schemas (User, Participant, Invoice, Session, Attendance, etc.)
- Request/response bodies for every endpoint
- Error responses, validation errors, rate limit documentation
- Bearer token auth scheme documented
- 2401 lines of production-ready API docs
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>