feat: 可配置方案引擎通用原型（Mock 检索）

需求未定稿时可先跑通追问/判定/出方案；槽位与判定树外置 YAML，后续可接 Ragflow。

Co-authored-by: haijd <haijd@163.com>

engine/app/main.py

@@ -0,0 +1,75 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+from fastapi import FastAPI, HTTPException
+from fastapi.responses import FileResponse
+from fastapi.staticfiles import StaticFiles
+from pydantic import BaseModel, Field
+
+from .config_loader import business_config
+from .scheme_engine import create_session, handle_message
+from .settings import settings
+
+STATIC_DIR = Path(__file__).resolve().parent / "static"
+
+app = FastAPI(title=settings.app_name, version="0.1.0")
+app.mount("/static", StaticFiles(directory=STATIC_DIR), name="static")
+
+
+class ChatRequest(BaseModel):
+    message: str = Field(..., min_length=1)
+    session_id: str | None = None
+
+
+class SessionResponse(BaseModel):
+    session_id: str
+
+
+@app.get("/")
+def index() -> FileResponse:
+    return FileResponse(STATIC_DIR / "index.html")
+
+
+@app.get("/api/health")
+def health() -> dict:
+    return {
+        "ok": True,
+        "app": settings.app_name,
+        "retrieval_backend": settings.retrieval_backend,
+        "slots": len(business_config.slots),
+        "rules": len(business_config.rules),
+    }
+
+
+@app.get("/api/config/slots")
+def get_slots() -> dict:
+    return business_config.slots_doc
+
+
+@app.get("/api/config/decision-tree")
+def get_tree() -> dict:
+    return business_config.tree_doc
+
+
+@app.post("/api/admin/reload-config")
+def reload_config() -> dict:
+    business_config.reload()
+    return {"reloaded": True, "slots": len(business_config.slots), "rules": len(business_config.rules)}
+
+
+@app.post("/api/sessions", response_model=SessionResponse)
+def api_create_session() -> SessionResponse:
+    session = create_session()
+    return SessionResponse(session_id=session.id)
+
+
+@app.post("/api/chat")
+def api_chat(body: ChatRequest) -> dict:
+    session_id = body.session_id
+    if not session_id:
+        session_id = create_session().id
+    try:
+        return handle_message(session_id, body.message)
+    except KeyError:
+        raise HTTPException(status_code=404, detail="session_not_found") from None