feat: implement QuestionBank CRUD with pagination and template query

- Add pagination support to findAll (page, limit query params)
- Add findByTemplateId method to service
- Add GET /by-template/:templateId endpoint to controller
- Service already includes CRUD for QuestionBank and QuestionBankItem

server/src/api/api.service.ts

@@ -0,0 +1,48 @@
+import { Injectable } from '@nestjs/common';
+import { ChatOpenAI } from '@langchain/openai';
+import { ModelConfig } from '../types';
+import { I18nService } from '../i18n/i18n.service';
+
+@Injectable()
+export class ApiService {
+  constructor(private i18nService: I18nService) {}
+
+  // Simple health check method
+  healthCheck() {
+    return { status: 'ok', message: 'API is healthy' };
+  }
+
+  async getChatCompletion(
+    prompt: string,
+    modelConfig: ModelConfig,
+  ): Promise<string> {
+    // API key is optional - allows local models
+
+    try {
+      const llm = this.createLLM(modelConfig);
+      const response = await llm.invoke(prompt);
+      return response.content.toString();
+    } catch (error) {
+      console.error('LangChain call failed:', error);
+      if (error.message?.includes('401')) {
+        throw new Error(this.i18nService.getMessage('invalidApiKey'));
+      }
+      throw new Error(
+        this.i18nService.formatMessage('apiCallFailed', {
+          message: error.message,
+        }),
+      );
+    }
+  }
+
+  private createLLM(modelConfig: ModelConfig): ChatOpenAI {
+    return new ChatOpenAI({
+      temperature: 0.7,
+      apiKey: modelConfig.apiKey,
+      modelName: modelConfig.modelId,
+      configuration: {
+        baseURL: modelConfig.baseUrl || 'http://localhost:11434/v1',
+      },
+    });
+  }
+}