feat: 后端基础设施 — LLM工厂/Embedding工厂/验证客户端/会话持久化

- backend/llm.py: 支持 OpenAI 兼容 API 与 Ollama 本地模型切换
- backend/embeddings.py: 支持云端与本地嵌入模型（sentence-transformers）
- backend/validation.py: FastAPI 验证服务 HTTP 客户端
- backend/session.py: JSON 文件会话管理（创建/加载/保存/列表/删除）
- .env.example: 完整环境变量模板
- requirements.txt: 所有 Python 依赖声明

backend/session.py

@@ -0,0 +1,112 @@
+"""多会话持久化管理模块。
+
+每个会话对应一个独立的 JSON 文件存储在 ./sessions/ 目录下。
+"""
+
+import json
+import os
+import uuid
+from datetime import datetime, timezone
+from pathlib import Path
+from typing import Optional
+
+from dotenv import load_dotenv
+
+load_dotenv()
+
+SESSIONS_DIR = Path(os.getenv("SESSIONS_DIR", "./sessions"))
+
+
+def _ensure_dir():
+    SESSIONS_DIR.mkdir(parents=True, exist_ok=True)
+
+
+def _session_path(session_id: str) -> Path:
+    return SESSIONS_DIR / f"{session_id}.json"
+
+
+def generate_session_id() -> str:
+    return uuid.uuid4().hex[:12]
+
+
+def create_session(name: str = "", agent_state: Optional[dict] = None) -> dict:
+    """创建新会话，返回会话元数据。"""
+    _ensure_dir()
+    sid = generate_session_id()
+    now = _now_iso()
+    data = {
+        "session_id": sid,
+        "session_name": name or f"新建报表 {now[:10]}",
+        "created_at": now,
+        "updated_at": now,
+        "agent_state": agent_state or {},
+    }
+    with open(_session_path(sid), "w", encoding="utf-8") as f:
+        json.dump(data, f, ensure_ascii=False, indent=2)
+    return data
+
+
+def load_session(session_id: str) -> Optional[dict]:
+    """按 ID 加载会话数据。未找到则返回 None。"""
+    _ensure_dir()
+    fp = _session_path(session_id)
+    if not fp.exists():
+        return None
+    with open(fp, "r", encoding="utf-8") as f:
+        return json.load(f)
+
+
+def save_session(session_id: str, agent_state: dict, session_name: str = ""):
+    """将会话状态保存（更新）至磁盘。"""
+    _ensure_dir()
+    fp = _session_path(session_id)
+    data = {}
+    if fp.exists():
+        with open(fp, "r", encoding="utf-8") as f:
+            data = json.load(f)
+
+    data["session_id"] = session_id
+    if session_name:
+        data["session_name"] = session_name
+    if not data.get("session_name"):
+        data["session_name"] = f"报表 {data.get('created_at', _now_iso())[:10]}"
+    data["updated_at"] = _now_iso()
+    if not data.get("created_at"):
+        data["created_at"] = data["updated_at"]
+    data["agent_state"] = agent_state
+
+    with open(fp, "w", encoding="utf-8") as f:
+        json.dump(data, f, ensure_ascii=False, indent=2)
+
+
+def list_all_sessions() -> list[dict]:
+    """列出所有历史会话（仅摘要，不含完整 agent_state）。"""
+    _ensure_dir()
+    sessions = []
+    for fp in sorted(SESSIONS_DIR.glob("*.json"), key=os.path.getmtime, reverse=True):
+        try:
+            with open(fp, "r", encoding="utf-8") as f:
+                data = json.load(f)
+            sessions.append({
+                "session_id": data.get("session_id", fp.stem),
+                "session_name": data.get("session_name", fp.stem),
+                "created_at": data.get("created_at", ""),
+                "updated_at": data.get("updated_at", ""),
+            })
+        except (json.JSONDecodeError, KeyError):
+            continue
+    return sessions
+
+
+def delete_session(session_id: str) -> bool:
+    """按 ID 删除会话文件。"""
+    _ensure_dir()
+    fp = _session_path(session_id)
+    if fp.exists():
+        fp.unlink()
+        return True
+    return False
+
+
+def _now_iso() -> str:
+    return datetime.now(timezone.utc).isoformat()