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/dist/admin/admin-config.service.d.ts

@@ -0,0 +1,54 @@
+import { PrismaService } from '../prisma/prisma.service';
+export declare class AdminConfigService {
+    private readonly prisma;
+    constructor(prisma: PrismaService);
+    findAll(category?: string): Promise<{
+        id: string;
+        key: string;
+        value: string;
+        category: string;
+        label: string;
+        description: string | null;
+        type: string;
+        isSecret: boolean;
+        updatedAt: Date;
+        updatedBy: string | null;
+    }[]>;
+    findOne(key: string): Promise<{
+        id: string;
+        key: string;
+        value: string;
+        category: string;
+        label: string;
+        description: string | null;
+        type: string;
+        isSecret: boolean;
+        updatedAt: Date;
+        updatedBy: string | null;
+    } | null>;
+    upsert(key: string, data: any, updatedBy: string): Promise<{
+        id: string;
+        key: string;
+        value: string;
+        category: string;
+        label: string;
+        description: string | null;
+        type: string;
+        isSecret: boolean;
+        updatedAt: Date;
+        updatedBy: string | null;
+    }>;
+    delete(key: string): Promise<{
+        id: string;
+        key: string;
+        value: string;
+        category: string;
+        label: string;
+        description: string | null;
+        type: string;
+        isSecret: boolean;
+        updatedAt: Date;
+        updatedBy: string | null;
+    }>;
+    getByCategory(): Promise<Record<string, any[]>>;
+}