- Add SessionEventRepository interface for append-only event storage
- Implement PostgreSQL session_event_repo with immutable event log
- Add database migration for session_events table with indexes
- Record events for keygen and sign session creation
- Record events for signing-config APIs (set, update, clear)
- Wire up sessionEventRepo in main.go and account handler
- Update API documentation with event sourcing design
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
