feat: P2-15 cron-based DingTalk/WeCom sync with logging and incremental sync

- Add @nestjs/schedule dependency
- Create SyncLog entity (platform, syncType, status, recordsCount, errorMessage, timestamps)
- Create SyncState entity for tracking lastSyncAt per platform
- SyncService with @Cron('0 2 * * *') daily scheduled sync
- Incremental sync: tracks last_sync_at per platform, passes to integration APIs
- Full/incremental mode determined by existence of prior sync
- SyncController: POST /sync/trigger (manual), GET /sync/logs (history)
- Register SyncModule in AppModule with ScheduleModule

apps/server/src/entities/index.ts

@@ -20,3 +20,5 @@ export { ClassTeacher, TeacherRoleType } from './class-teacher.entity';
 export { ClassSchedule, ScheduleType } from './class-schedule.entity';
 export { AttendanceRecord } from './attendance-record.entity';
 export { DingAttendanceRaw } from './ding-attendance-raw.entity';
+export { SyncLog } from './sync-log.entity';
+export { SyncState } from './sync-state.entity';