fix(aia,ssa,asl,infra): harden SSE transport and stabilize attachment context

Deliver SSE protocol hardening for SAE/HTTP2 paths, add graceful shutdown health behavior, and improve SSA retry UX for transient stream failures. For AIA, persist attachment extraction results in database with cache read-through fallback, plus production cache safety guard to prevent memory-cache drift in multi-instance deployments; also restore ASL SR page scrolling behavior.

Made-with: Cursor

frontend-v2/nginx.conf

@@ -17,6 +17,13 @@ http {
     include       /etc/nginx/mime.types;
     default_type  application/octet-stream;
 
+    # WebSocket / SSE 兼容：仅当请求包含 Upgrade 头时设置 Connection: upgrade
+    # SSE 请求无 Upgrade 头 → Connection 为空，避免 HTTP/2 帧协议错误
+    map $http_upgrade $connection_upgrade {
+        default upgrade;
+        ''      '';
+    }
+
     # ⚠️ 文件上传大小限制（默认只有 1MB，太小会导致 413 错误）
     client_max_body_size 50M;  # 允许上传最大 50MB 文件
 
@@ -144,14 +151,15 @@ http {
             proxy_send_timeout 300s;
             proxy_read_timeout 300s;
             
-            # 缓冲配置
-            proxy_buffering off;  # 关闭缓冲（实时流式响应）
-            proxy_request_buffering off;  # 支持大文件上传
+            # 缓冲配置（SSE 流式响应必须关闭所有缓冲/缓存）
+            proxy_buffering off;
+            proxy_cache off;
+            proxy_request_buffering off;
             
-            # WebSocket 支持（如果后续需要）
+            # WebSocket + SSE 兼容
             proxy_http_version 1.1;
             proxy_set_header Upgrade $http_upgrade;
-            proxy_set_header Connection "upgrade";
+            proxy_set_header Connection $connection_upgrade;
             
             # 错误处理
             proxy_next_upstream error timeout invalid_header http_500 http_502 http_503;