feat: connect chat endpoint to verified retrieval

backend/app/api/routes/chat.py

@@ -1,15 +1,34 @@
-from fastapi import APIRouter
+from fastapi import APIRouter, Depends
+from sqlalchemy.orm import Session
 
+from app.db.session import get_db
 from app.schemas.chat import ChatRequest, ChatResponse
+from app.schemas.retrieval import RetrievalRequest
+from app.services.retrieval import retrieve
 
 router = APIRouter(tags=["chat"])
 
 
 @router.post("/chat", response_model=ChatResponse)
-def chat(payload: ChatRequest) -> ChatResponse:
-    # TODO: retrieval -> context assembly -> LLM -> evidence/source checks
+def chat(payload: ChatRequest, db: Session = Depends(get_db)) -> ChatResponse:
+    """运行可复核的关键词检索；模型未配置时绝不伪造生成式回答。"""
+    result = retrieve(db, RetrievalRequest(query=payload.question, top_k=5))
+    if result.status == "no_evidence":
+        return ChatResponse(
+            answer="未检索到可用的已核验知识依据。请补充更具体的关键词，或由管理员补充并审核资料。",
+            status="no_evidence",
+            sources=[],
+            retrieval_run_id=result.run_id,
+        )
     return ChatResponse(
-        answer=f"已收到问题：{payload.question}。当前为基础壳子，尚未接入知识检索或大语言模型。",
-        status="stub",
-        sources=[],
+        answer=(
+            f"已检索到 {result.total_hits} 条已核验知识依据。当前尚未配置大语言模型，"
+            "因此系统仅返回可追溯证据，不生成或改写答案。"
+        ),
+        status="retrieval_only",
+        sources=[
+            {"title": hit.title, "source_url": hit.source_url}
+            for hit in result.results
+        ],
+        retrieval_run_id=result.run_id,
     )