Introduces incremental API improvements for custom launcher support
(mhf-iel, stratic-dev's Rust launcher):
- Standardize all error responses to JSON envelopes with error/message
- Add Bearer token auth middleware for v2 routes (legacy body-token preserved)
- Add `returning` (>90d inactive) and `courses` fields to auth response
- Add /v2/ route prefix with HTTP method enforcement
- Add GET /v2/server/status for MezFes, featured weapon, and event status
- Add APIEventRepo for read-only event data access
Closes#44
