Implement version-based optimistic locking to prevent concurrent update conflicts
when multiple parties simultaneously report completion during keygen operations.
Changes:
- Add version column to mpc_sessions table (migration 004)
- Add Version field to MPCSession entity
- Define ErrOptimisticLockConflict error
- Update SessionPostgresRepo.Update() to check version and increment on success
- Add automatic retry logic (max 3 attempts) to ReportCompletionUseCase
- Update Save and all query methods (FindByStatus, FindExpired, etc.) to handle version field
This replaces pessimistic locking (FOR UPDATE) with optimistic locking using
the industry-standard pattern: WHERE version = $n and checking rowsAffected.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
