fix: 分块大小减至5MB + Edge端chunk_size变更自动清理 + 断点续传防护

NAS dispatcher:
- CHUNK_SIZE 10MB→5MB（~1Mbps上行带宽下可靠传输）
- chunk上传timeout (30,120)→(60,180)（增加连接和读取余量）
- _query_uploaded_chunks 返回 (set, edge_total) 元组
- expected_total != edge_total 时跳过断点续传（防止chunk_size变更导致文件损坏）

Edge api_gateway:
- upload_chunk 检测 total_chunks 变更，自动清理旧分块
- 防止不同chunk_size的旧分块与新分块混合导致assemble后文件损坏

fam-edge/src/fam_edge/api_gateway/api_gateway.py

@@ -168,11 +168,26 @@ def upload_chunk():
     chunk_path = os.path.join(d, f"chunk_{chunk_index:04d}")
 
     try:
+        # 检查 total_chunks 是否变化（chunk_size 变更导致），自动清理旧分块
+        meta_path = os.path.join(d, "meta.json")
+        if total_chunks and os.path.exists(meta_path):
+            import json
+            try:
+                with open(meta_path) as mf:
+                    old_meta = json.load(mf)
+                if old_meta.get('total_chunks') and old_meta['total_chunks'] != total_chunks:
+                    logger.warning(f"[task_id={task_id}] total_chunks 变更 "
+                                   f"({old_meta['total_chunks']}→{total_chunks})，清理旧分块")
+                    for fn in os.listdir(d):
+                        if fn.startswith('chunk_'):
+                            os.remove(os.path.join(d, fn))
+            except (json.JSONDecodeError, IOError):
+                pass
+
         chunk_file.save(chunk_path)
         size_kb = os.path.getsize(chunk_path) / 1024
 
-        # 写元数据（首次上传时）
-        meta_path = os.path.join(d, "meta.json")
+        # 写/更新元数据
         if not os.path.exists(meta_path):
             import json
             meta = {"filename": filename, "total_chunks": total_chunks}