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>

CLAUDE.md

@@ -78,6 +78,13 @@ src/
 - Covers all 31 TS/TSX files + 4 CSS files
 - Update the index when adding/removing/renaming files or exports
 
+## Database Migrations
+- **Never drop/recreate the database** — admin data (photos, edits, registrations) lives there
+- Schema changes go through versioned migrations in `src/lib/db.ts` (`migrations` array)
+- Add a new entry with the next version number; never modify existing migrations
+- Migrations run automatically on server start via `runMigrations()` and are tracked in the `_migrations` table
+- Use `CREATE TABLE IF NOT EXISTS` and column-existence checks (`PRAGMA table_info`) for safety
+
 ## Git
 - Remote: Gitea at `git.dolgolyov-family.by`
 - User: diana.dolgolyova