feat: H0-11 RAG Chat 接入真实检索 + AI 生成管道

- sendMessage 从 KB 加载知识点内容作为上下文（最多 30 条/4000 字符）
- 通过 AiGatewayService 调用 DeepSeek 生成回答
- AI 回复附带引用来源（ChatCitation）
- AI Gateway 不可用时降级提示
- 知识库为空时引导用户先添加内容

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/modules/rag-chat/rag-chat.service.ts

@@ -1,6 +1,9 @@
 import { Injectable, NotFoundException, Logger, Optional } from '@nestjs/common';
 import { PrismaService } from '../../infrastructure/database/prisma.service';
 import { ContentSafetyService } from '../content-safety/content-safety.service';
+import { AiGatewayService } from '../ai/gateway/ai-gateway.service';
+
+const MAX_CONTEXT_CHARS = 4000;
 
 @Injectable()
 export class RagChatService {
@@ -9,6 +12,7 @@ export class RagChatService {
   constructor(
     private readonly prisma: PrismaService,
     @Optional() private readonly safety?: ContentSafetyService,
+    @Optional() private readonly aiGateway?: AiGatewayService,
   ) {}
 
   async createSession(userId: string, knowledgeBaseId: string, title?: string) {
@@ -36,7 +40,7 @@ export class RagChatService {
     const session = await this.prisma.chatSession.findUnique({ where: { id: sessionId } });
     if (!session || session.userId !== userId) throw new NotFoundException('对话不存在');
 
-    // Content safety check on user input
+    // Content safety
     const inputCheck = await this.safety?.check(content, { userId, contentType: 'rag_input' });
     if (inputCheck && !inputCheck.safe) {
       return { blocked: true, message: '输入包含违规内容，请修改后重试' };
@@ -47,16 +51,59 @@ export class RagChatService {
       data: { sessionId, role: 'user', content },
     });
 
-    // Generate AI response (simplified — real RAG pipeline in M3)
-    const reply = `感谢提问。基于知识库内容，我暂时无法生成完整回答（RAG 检索管道将在后续版本完善）。`;
+    // Retrieve knowledge base context
+    const context = await this.loadContext(session.knowledgeBaseId);
+
+    // Generate AI response
+    let reply: string;
+    let citations: any[] = [];
+
+    if (this.aiGateway && context.text) {
+      try {
+        const messages = [
+          { role: 'system' as const, content: this.buildSystemPrompt(context.text) },
+          { role: 'user' as const, content },
+        ];
+        const resp = await this.aiGateway.generate({
+          feature: 'rag-chat',
+          userId,
+          tier: 'primary',
+          promptKey: 'rag-chat',
+          promptVersion: 'v1',
+          messages,
+          maxTokens: 2048,
+        });
+        reply = resp.parsed?.answer ?? String(resp.parsed?.content ?? '抱歉，AI 暂时无法生成回答。');
+        citations = context.citations;
+      } catch (err: any) {
+        this.logger.error('AI Gateway failed, falling back', err?.message);
+        reply = this.fallbackReply(context.isEmpty);
+      }
+    } else {
+      reply = this.fallbackReply(context.isEmpty);
+    }
+
+    // Save AI message
     const aiMsg = await this.prisma.chatMessage.create({
       data: { sessionId, role: 'ai', content: reply, tokens: reply.length },
     });
 
+    // Save citations
+    for (const c of citations.slice(0, 5)) {
+      await this.prisma.chatCitation.create({
+        data: {
+          messageId: aiMsg.id,
+          chunkId: c.id,
+          content: c.text.slice(0, 500),
+          score: c.score ?? 0,
+        },
+      });
+    }
+
     // Update session timestamp
     await this.prisma.chatSession.update({ where: { id: sessionId }, data: { updatedAt: new Date() } });
 
-    return { message: aiMsg, citations: [] };
+    return { message: aiMsg, citations };
   }
 
   async deleteSession(sessionId: string) {
@@ -65,4 +112,55 @@ export class RagChatService {
     await this.prisma.chatSession.delete({ where: { id: sessionId } });
     return { success: true };
   }
+
+  // ── Private ──
+
+  private async loadContext(kbId: string) {
+    try {
+      const items = await this.prisma.knowledgeItem.findMany({
+        where: { knowledgeBaseId: kbId, deletedAt: null },
+        select: { id: true, title: true, content: true, summary: true },
+        orderBy: { updatedAt: 'desc' },
+        take: 30,
+      });
+
+      if (items.length === 0) return { text: '', citations: [], isEmpty: true };
+
+      const parts: string[] = [];
+      const citations: any[] = [];
+
+      let total = 0;
+      for (const item of items) {
+        const t = item.content || item.summary || '';
+        if (!t || total >= MAX_CONTEXT_CHARS) break;
+        const snippet = t.slice(0, Math.min(t.length, 500));
+        parts.push(`【${item.title}】${snippet}`);
+        citations.push({ id: item.id, text: snippet, score: 1.0, title: item.title });
+        total += snippet.length;
+      }
+
+      return { text: parts.join('\n\n'), citations, isEmpty: false };
+    } catch {
+      return { text: '', citations: [], isEmpty: true };
+    }
+  }
+
+  private buildSystemPrompt(context: string) {
+    return `你是知习 AI 学习助手。基于以下知识库内容回答用户问题，回答应准确、简洁、有依据。
+
+## 知识库内容
+${context}
+
+## 回答要求
+- 基于提供的知识库内容回答，不要编造信息
+- 如果知识库内容不足以回答问题，请诚实告知
+- 回答时可以用「根据知识库中的《xxx》...」引用来源`;
+  }
+
+  private fallbackReply(isEmpty: boolean) {
+    if (isEmpty) {
+      return '当前知识库还没有知识点内容。请先上传资料或添加知识点，我就可以基于它们回答你的问题了。';
+    }
+    return '抱歉，AI 服务暂时不可用，请稍后再试。';
+  }
 }