refactor(backend): incremental architecture evolution (Task 19)

- Add common/ layer for shared capabilities (LLM, RAG, document, middleware)
- Add legacy/ layer for existing business code
- Move files to new structure (controllers, routes, services)
- Update index.ts for new route registration
- System remains fully functional

backend/src/common/llm/adapters/DeepSeekAdapter.ts

@@ -0,0 +1,150 @@
+import axios from 'axios';
+import { ILLMAdapter, Message, LLMOptions, LLMResponse, StreamChunk } from './types.js';
+import { config } from '../../../config/env.js';
+
+export class DeepSeekAdapter implements ILLMAdapter {
+  modelName: string;
+  private apiKey: string;
+  private baseURL: string;
+
+  constructor(modelName: string = 'deepseek-chat') {
+    this.modelName = modelName;
+    this.apiKey = config.deepseekApiKey || '';
+    this.baseURL = 'https://api.deepseek.com/v1';
+
+    if (!this.apiKey) {
+      throw new Error('DeepSeek API key is not configured');
+    }
+  }
+
+  // 非流式调用
+  async chat(messages: Message[], options?: LLMOptions): Promise<LLMResponse> {
+    try {
+      const response = await axios.post(
+        `${this.baseURL}/chat/completions`,
+        {
+          model: this.modelName,
+          messages: messages,
+          temperature: options?.temperature ?? 0.7,
+          max_tokens: options?.maxTokens ?? 2000,
+          top_p: options?.topP ?? 0.9,
+          stream: false,
+        },
+        {
+          headers: {
+            'Content-Type': 'application/json',
+            Authorization: `Bearer ${this.apiKey}`,
+          },
+          timeout: 180000, // 180秒超时（3分钟）- 稿件评估需要更长时间
+        }
+      );
+
+      const choice = response.data.choices[0];
+      
+      return {
+        content: choice.message.content,
+        model: response.data.model,
+        usage: {
+          promptTokens: response.data.usage.prompt_tokens,
+          completionTokens: response.data.usage.completion_tokens,
+          totalTokens: response.data.usage.total_tokens,
+        },
+        finishReason: choice.finish_reason,
+      };
+    } catch (error: unknown) {
+      console.error('DeepSeek API Error:', error);
+      if (axios.isAxiosError(error)) {
+        throw new Error(
+          `DeepSeek API调用失败: ${error.response?.data?.error?.message || error.message}`
+        );
+      }
+      throw error;
+    }
+  }
+
+  // 流式调用
+  async *chatStream(
+    messages: Message[],
+    options?: LLMOptions,
+    onChunk?: (chunk: StreamChunk) => void
+  ): AsyncGenerator<StreamChunk, void, unknown> {
+    try {
+      const response = await axios.post(
+        `${this.baseURL}/chat/completions`,
+        {
+          model: this.modelName,
+          messages: messages,
+          temperature: options?.temperature ?? 0.7,
+          max_tokens: options?.maxTokens ?? 2000,
+          top_p: options?.topP ?? 0.9,
+          stream: true,
+        },
+        {
+          headers: {
+            'Content-Type': 'application/json',
+            Authorization: `Bearer ${this.apiKey}`,
+          },
+          responseType: 'stream',
+          timeout: 60000,
+        }
+      );
+
+      const stream = response.data;
+      let buffer = '';
+
+      for await (const chunk of stream) {
+        buffer += chunk.toString();
+        const lines = buffer.split('\n');
+        buffer = lines.pop() || '';
+
+        for (const line of lines) {
+          const trimmedLine = line.trim();
+          if (!trimmedLine || trimmedLine === 'data: [DONE]') {
+            continue;
+          }
+
+          if (trimmedLine.startsWith('data: ')) {
+            try {
+              const jsonStr = trimmedLine.slice(6);
+              const data = JSON.parse(jsonStr);
+              
+              const choice = data.choices[0];
+              const content = choice.delta?.content || '';
+              
+              const streamChunk: StreamChunk = {
+                content: content,
+                done: choice.finish_reason === 'stop',
+                model: data.model,
+              };
+
+              if (choice.finish_reason === 'stop' && data.usage) {
+                streamChunk.usage = {
+                  promptTokens: data.usage.prompt_tokens,
+                  completionTokens: data.usage.completion_tokens,
+                  totalTokens: data.usage.total_tokens,
+                };
+              }
+
+              if (onChunk) {
+                onChunk(streamChunk);
+              }
+
+              yield streamChunk;
+            } catch (parseError) {
+              console.error('Failed to parse SSE data:', parseError);
+            }
+          }
+        }
+      }
+    } catch (error) {
+      console.error('DeepSeek Stream Error:', error);
+      if (axios.isAxiosError(error)) {
+        throw new Error(
+          `DeepSeek流式调用失败: ${error.response?.data?.error?.message || error.message}`
+        );
+      }
+      throw error;
+    }
+  }
+}
+