refactor(frontend): colocate admin components and split service layer

Move route-specific admin components from components/common/admin into
app/(main)/admin/<feature>/components/. Split AdminService god object into
domain services, fix UploadService to use BaseService, add AdminUploadService,
consolidate DB export into DbManageService, and migrate consumers to the
unified services entry.

frontend/lib/services/admin/template.service.ts

@@ -0,0 +1,26 @@
+import {BaseService} from '@/lib/services/core';
+import type {CreateTemplateRequest, Template, UpdateTemplateRequest} from './types';
+
+export class AdminTemplateService extends BaseService {
+  protected static readonly basePath = '/api/v1/admin';
+
+  static async listTemplates(): Promise<Template[]> {
+    return this.get<Template[]>('/templates');
+  }
+
+  static async getTemplate(key: string): Promise<Template> {
+    return this.get<Template>(`/templates/${key}`);
+  }
+
+  static async createTemplate(request: CreateTemplateRequest): Promise<Template> {
+    return this.post<Template>('/templates', request);
+  }
+
+  static async updateTemplate(key: string, request: UpdateTemplateRequest): Promise<Template> {
+    return this.put<Template>(`/templates/${key}`, request);
+  }
+
+  static async deleteTemplate(key: string): Promise<void> {
+    return this.delete<void>(`/templates/${key}`);
+  }
+}