Add applicationId to syncableEntity and fix syncApp deletion (#15170)

## Context
- All flatEntity should extend SyncableEntity
- SyncableEntity should now have applicationId and application relation
- Fix syncApp deletion, should now properly use migration v2 to delete
syncable entities

packages/twenty-server/src/engine/workspace-manager/workspace-sync/interfaces/syncable-entity.interface.ts

@@ -1,4 +1,6 @@
-import { Column, Index } from 'typeorm';
+import { Column, Index, JoinColumn, ManyToOne, Relation } from 'typeorm';
+
+import type { ApplicationEntity } from 'src/engine/core-modules/application/application.entity';
 
 @Index(['workspaceId', 'universalIdentifier'], {
   unique: true,
@@ -6,4 +8,14 @@ import { Column, Index } from 'typeorm';
 export abstract class SyncableEntity {
   @Column({ nullable: true, type: 'uuid' })
   universalIdentifier: string | null;
+
+  @Column({ nullable: true, type: 'uuid' })
+  applicationId: string | null;
+
+  @ManyToOne('ApplicationEntity', {
+    onDelete: 'CASCADE',
+    nullable: true,
+  })
+  @JoinColumn({ name: 'applicationId' })
+  application: Relation<ApplicationEntity>;
 }