fix: session persistence, multi-turn memory, OCR pipeline, download UX (v7)

- graph.stream() state fix: agent_state now properly accumulates node updates

- atomic session save (tempfile + os.replace)

- uploaded_file_path injection for OcrExtractor + annotation_detector

- download section always visible; refreshFromApi auto-reloads after generation

- node_start/complete unfiltered for full progress visibility

- modification_request without status=='pass' check

backend/session.py

@@ -6,6 +6,7 @@
 import json
 import os
 import uuid
+import tempfile
 from datetime import datetime, timezone
 from pathlib import Path
 from typing import Optional
@@ -64,7 +65,7 @@ def load_session(session_id: str) -> Optional[dict]:
 
 
 def save_session(session_id: str, agent_state: dict, session_name: str = ""):
-    """将会话状态保存（更新）至磁盘。"""
+    """将会话状态原子保存至磁盘（temp file + rename，避免崩溃时截断）。"""
     _ensure_dir()
     fp = _session_path(session_id)
     data = {}
@@ -82,8 +83,19 @@ def save_session(session_id: str, agent_state: dict, session_name: str = ""):
         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)
+    # 原子写入：先写临时文件，再 replace，避免崩溃时截断 JSON
+    tmp = tempfile.NamedTemporaryFile(
+        mode="w", suffix=".json", delete=False,
+        dir=SESSIONS_DIR, encoding="utf-8",
+    )
+    try:
+        json.dump(data, tmp, ensure_ascii=False, indent=2)
+        tmp.close()
+        os.replace(tmp.name, str(fp))
+    except Exception:
+        tmp.close()
+        Path(tmp.name).unlink(missing_ok=True)
+        raise
 
 
 def get_session_state(session_id: str) -> Optional[dict]: