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/sync/sync.module.ts

@@ -0,0 +1,17 @@
+import { Module } from '@nestjs/common';
+import { ScheduleModule } from '@nestjs/schedule';
+import { TypeOrmModule } from '@nestjs/typeorm';
+import { SyncLog, SyncState } from '../entities';
+import { SyncService } from './sync.service';
+import { SyncController } from './sync.controller';
+
+@Module({
+  imports: [
+    ScheduleModule.forRoot(),
+    TypeOrmModule.forFeature([SyncLog, SyncState]),
+  ],
+  controllers: [SyncController],
+  providers: [SyncService],
+  exports: [SyncService],
+})
+export class SyncModule {}