P3-02-03-04: audit log, batch ops, transactions

P3-02: audit-log.entity + service, manual logging in controller
  (startSession, submitAnswer, deleteSession, review, forceEnd)
P3-03: POST batch-delete, POST batch-export endpoints + service methods
P3-04: DataSource.transaction for deleteSession + reviewAssessment,
  graph state cleanup on session delete

server/src/assessment/entities/audit-log.entity.ts

@@ -0,0 +1,28 @@
+import { Entity, PrimaryGeneratedColumn, Column, CreateDateColumn } from 'typeorm';
+
+@Entity('audit_logs')
+export class AuditLog {
+  @PrimaryGeneratedColumn('uuid')
+  id: string;
+
+  @Column({ name: 'user_id', type: 'text' })
+  userId: string;
+
+  @Column({ name: 'tenant_id', nullable: true, type: 'text' })
+  tenantId: string;
+
+  @Column({ type: 'varchar', length: 50 })
+  action: string;
+
+  @Column({ name: 'resource_type', type: 'varchar', length: 50 })
+  resourceType: string;
+
+  @Column({ name: 'resource_id', nullable: true, type: 'text' })
+  resourceId: string;
+
+  @Column({ type: 'simple-json', nullable: true })
+  details: any;
+
+  @CreateDateColumn({ name: 'created_at' })
+  createdAt: Date;
+}