feat: complete admin backend controllers and services

- AdminPlansController & Service (CRUD, reorder)
- AdminPaymentMethodsController & Service (CRUD, reorder)
- AdminPaymentsController & Service (verify, reject, pending count)
- AdminUsersController & Service (search, suspend, grant pro access, stats)
- AdminConfigController & Service (dynamic config management)
- Wire all controllers into AdminModule
- Import AdminModule in AppModule

Admin API Routes:
- GET/POST/PUT/DELETE /admin/plans
- GET/POST/PUT/DELETE /admin/payment-methods
- GET /admin/payments (with status filter)
- POST /admin/payments/:id/verify
- POST /admin/payments/:id/reject
- GET /admin/users (with search)
- POST /admin/users/:id/grant-pro
- GET/POST/DELETE /admin/config

All routes protected by AuthGuard + AdminGuard

apps/api/src/admin/admin.module.ts

@@ -0,0 +1,45 @@
+import { Module } from '@nestjs/common';
+import { PrismaModule } from '../prisma/prisma.module';
+import { AdminGuard } from './guards/admin.guard';
+
+// Controllers
+import { AdminPlansController } from './admin-plans.controller';
+import { AdminPaymentMethodsController } from './admin-payment-methods.controller';
+import { AdminPaymentsController } from './admin-payments.controller';
+import { AdminUsersController } from './admin-users.controller';
+import { AdminConfigController } from './admin-config.controller';
+
+// Services
+import { AdminPlansService } from './admin-plans.service';
+import { AdminPaymentMethodsService } from './admin-payment-methods.service';
+import { AdminPaymentsService } from './admin-payments.service';
+import { AdminUsersService } from './admin-users.service';
+import { AdminConfigService } from './admin-config.service';
+
+@Module({
+  imports: [PrismaModule],
+  controllers: [
+    AdminPlansController,
+    AdminPaymentMethodsController,
+    AdminPaymentsController,
+    AdminUsersController,
+    AdminConfigController,
+  ],
+  providers: [
+    AdminGuard,
+    AdminPlansService,
+    AdminPaymentMethodsService,
+    AdminPaymentsService,
+    AdminUsersService,
+    AdminConfigService,
+  ],
+  exports: [
+    AdminGuard,
+    AdminPlansService,
+    AdminPaymentMethodsService,
+    AdminPaymentsService,
+    AdminUsersService,
+    AdminConfigService,
+  ],
+})
+export class AdminModule {}