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/sync-state.entity.ts

@@ -0,0 +1,11 @@
+import { Entity, PrimaryColumn, Column } from 'typeorm';
+import type { SyncPlatform } from './sync-log.entity';
+
+@Entity('sync_state')
+export class SyncState {
+  @PrimaryColumn({ type: 'varchar', length: 20 })
+  platform: SyncPlatform;
+
+  @Column({ name: 'last_sync_at', type: 'datetime', nullable: true })
+  lastSyncAt: Date | null;
+}