feat: add audit log system with comprehensive action tracking and user activity monitoring

Introduce AuditLog model to track system-wide user actions including authentication, course management, file operations, and user account changes. Add AuditAction enum with 17 action types (CREATE, UPDATE, DELETE, LOGIN, LOGOUT, ENROLL, UNENROLL, SUBMIT_QUIZ, APPROVE_COURSE, REJECT_COURSE, UPLOAD_FILE, DELETE_FILE, CHANGE_PASSWORD, RESET_PASSWORD, VERIFY_EMAIL, DEACTIVATE_USER, ACTIVATE_USER). Include fields

Backend/prisma/schema.prisma

@@ -64,6 +64,7 @@ model User {
   updated_withdrawals   WithdrawalRequest[] @relation("WithdrawalUpdater")
   submitted_approvals   CourseApproval[]    @relation("ApprovalSubmitter")
   reviewed_approvals    CourseApproval[]    @relation("ApprovalReviewer")
+  audit_logs            AuditLog[]
 
   @@index([username])
   @@index([email])
@@ -608,3 +609,52 @@ model WithdrawalRequest {
   @@index([instructor_id, status])
   @@map("withdrawal_requests")
 }
+
+// ============================================
+// Audit Log System
+// ============================================
+
+enum AuditAction {
+  CREATE
+  UPDATE
+  DELETE
+  LOGIN
+  LOGOUT
+  ENROLL
+  UNENROLL
+  SUBMIT_QUIZ
+  APPROVE_COURSE
+  REJECT_COURSE
+  UPLOAD_FILE
+  DELETE_FILE
+  CHANGE_PASSWORD
+  RESET_PASSWORD
+  VERIFY_EMAIL
+  DEACTIVATE_USER
+  ACTIVATE_USER
+}
+
+model AuditLog {
+  id          Int         @id @default(autoincrement())
+  user_id     Int?
+  action      AuditAction
+  entity_type String      @db.VarChar(100) // Course, User, Lesson, Quiz, etc.
+  entity_id   Int?
+  old_value   Json?       // ค่าเดิม (สำหรับ UPDATE/DELETE)
+  new_value   Json?       // ค่าใหม่ (สำหรับ CREATE/UPDATE)
+  ip_address  String?     @db.VarChar(45) // รองรับ IPv6
+  user_agent  String?     @db.Text
+  metadata    Json?       // ข้อมูลเพิ่มเติม เช่น request_id, session_id
+  created_at  DateTime    @default(now())
+
+  user User? @relation(fields: [user_id], references: [id], onDelete: SetNull)
+
+  @@index([user_id])
+  @@index([action])
+  @@index([entity_type])
+  @@index([entity_type, entity_id])
+  @@index([created_at])
+  @@index([user_id, created_at])
+  @@index([action, created_at])
+  @@map("audit_logs")
+}