feat: add groupId support and redesign schedule GroupView hierarchy

- Add groupId field to ScheduleClass for admin-defined group identity
- Add versioned DB migration system (replaces initTables) to prevent data loss
- Redesign GroupView: Trainer → Class Type → Group → Datetimes hierarchy
- Group datetimes by day, merge days with identical time sets
- Auto-assign groupIds to legacy schedule entries in admin
- Add mc_registrations CRUD to db.ts

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/types/content.ts

@@ -55,6 +55,7 @@ export interface ScheduleClass {
   level?: string;
   hasSlots?: boolean;
   recruiting?: boolean;
+  groupId?: string;
 }
 
 export interface ScheduleDay {