refactor: simplify instructor search to query all instructors without course-specific filtering

Backend/src/controllers/CoursesInstructorController.ts

@@ -198,19 +198,18 @@ export class CoursesInstructorController {
     }
 
     /**
-     * ค้นหาผู้สอนสำหรับเพิ่มเข้าคอร์ส
-     * Search instructors to add to course
-     * @param courseId - รหัสคอร์ส / Course ID
+     * ค้นหาผู้สอนทั้งหมดในระบบ
+     * Search all instructors in database
      * @param query - คำค้นหา (email หรือ username) / Search query (email or username)
      */
-    @Get('{courseId}/search-instructors')
+    @Get('search-instructors')
     @Security('jwt', ['instructor'])
     @SuccessResponse('200', 'Instructors found')
     @Response('401', 'Invalid or expired token')
-    public async searchInstructors(@Request() request: any, @Path() courseId: number, @Query() query: string): Promise<SearchInstructorResponse> {
+    public async searchInstructors(@Request() request: any, @Query() query: string): Promise<SearchInstructorResponse> {
         const token = request.headers.authorization?.replace('Bearer ', '');
         if (!token) throw new ValidationError('No token provided');
-        return await CoursesInstructorService.searchInstructors({ token, course_id: courseId, query });
+        return await CoursesInstructorService.searchInstructors({ token, query });
     }
 
     /**

Backend/src/services/CoursesInstructor.service.ts

@@ -348,31 +348,16 @@ export class CoursesInstructorService {
 
     static async searchInstructors(input: SearchInstructorInput): Promise<SearchInstructorResponse> {
         try {
-            const decoded = jwt.verify(input.token, config.jwt.secret) as { id: number };
-            
-            // Validate user is instructor of this course
-            await this.validateCourseInstructor(input.token, input.course_id);
+            jwt.verify(input.token, config.jwt.secret) as { id: number };
 
-            // Get existing instructors of this course
-            const existingInstructors = await prisma.courseInstructor.findMany({
-                where: { course_id: input.course_id },
-                select: { user_id: true }
-            });
-            const existingUserIds = existingInstructors.map(i => i.user_id);
-
-            // Search users by email or username (only instructors role)
+            // Search all instructors by email or username
             const users = await prisma.user.findMany({
                 where: {
-                    AND: [
-                        {
-                            OR: [
-                                { email: { contains: input.query, mode: 'insensitive' } },
-                                { username: { contains: input.query, mode: 'insensitive' } },
-                            ]
-                        },
-                        { role: { code: 'instructor' } },
-                        { id: { notIn: existingUserIds } } // Exclude already added instructors
-                    ]
+                    OR: [
+                        { email: { contains: input.query, mode: 'insensitive' } },
+                        { username: { contains: input.query, mode: 'insensitive' } },
+                    ],
+                    role: { code: 'instructor' }
                 },
                 include: {
                     profile: true

Backend/src/types/CoursesInstructor.types.ts

@@ -97,7 +97,6 @@ export interface addinstructorCourse {
 export interface SearchInstructorInput {
     token: string;
     query: string;
-    course_id: number;
 }
 
 export interface SearchInstructorResult {