fix(api): 限制上传文件大小为8MB

- 后端新增中间件限制请求体最大为8MB，超出返回413错误
- nginx配置添加client_max_body_size为8m以支持8MB最大上传
- 更新favicon.svg图标样式
- 模型列表页默认分页大小由10改为50，提高一次加载条目数

backend/main.py

@@ -1,5 +1,6 @@
-from fastapi import FastAPI, Depends
+from fastapi import FastAPI, Depends, Request
 from fastapi.middleware.cors import CORSMiddleware
+from fastapi.responses import JSONResponse
 import os
 import logging
 from dotenv import load_dotenv
@@ -39,6 +40,22 @@ app = FastAPI(
     version="1.0.0"
 )
 
+# 限制请求体最大 8MB
+MAX_BODY_SIZE = 8 * 1024 * 1024  # 8MB
+
+
+@app.middleware("http")
+async def limit_body_size(request: Request, call_next):
+    content_length = request.headers.get("content-length")
+    if content_length and int(content_length) > MAX_BODY_SIZE:
+        return JSONResponse(
+            status_code=413,
+            content={"detail": "上传文件大小超过 8MB 限制"},
+        )
+    response = await call_next(request)
+    return response
+
+
 # Configure CORS
 app.add_middleware(
     CORSMiddleware,