fix: 修复 SQLAlchemy 初始化和 Flask-Login user_loader 配置

- models/__init__.py 改用 extensions.py 中的 db 实例
- 添加 Flask-Login user_loader 回调函数
- 添加 nginx 反向代理配置

修复了容器启动后 API 500 错误的问题

nginx.conf

@@ -0,0 +1,36 @@
+events {
+    worker_connections 1024;
+}
+
+http {
+    upstream pit_router {
+        server pit-router:9000;
+    }
+
+    server {
+        listen 80;
+        server_name _;
+
+        location / {
+            proxy_pass http://pit_router;
+            proxy_set_header Host $host;
+            proxy_set_header X-Real-IP $remote_addr;
+            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+            proxy_set_header X-Forwarded-Proto $scheme;
+        }
+
+        location /socket.io/ {
+            proxy_pass http://pit_router/socket.io/;
+            proxy_http_version 1.1;
+            proxy_set_header Upgrade $http_upgrade;
+            proxy_set_header Connection "upgrade";
+            proxy_set_header Host $host;
+            proxy_set_header X-Real-IP $remote_addr;
+            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        }
+
+        location /health {
+            proxy_pass http://pit_router/health;
+        }
+    }
+}