docs: clarify coverImage URL format and static asset paths

API.md

@@ -19,13 +19,15 @@
     "name": "我的项目",
     "description": "项目描述",
     "createdAt": "2026-05-25T10:00:00",
-    "coverImage": "https://...",   // nullable
+    "coverImage": "/uploads/covers/1/abc123.jpg",  // 相对路径，访问时需加域名
     "reportCount": 3,
     "todayNewReports": 1
   }
 ]
 ```
 
+> `coverImage` 字段为相对路径，完整访问地址为 `https://www.1415243231.top/uploads/covers/1/abc123.jpg`
+
 ---
 
 ### GET /projects/{id}
@@ -173,6 +175,16 @@ Content-Disposition: attachment; filename="filename.html"
 
 ---
 
+## 静态资源路径说明
+
+`coverImage` 字段为后端存储的相对路径，需拼接完整 URL 才能在浏览器中访问：
+
+| 字段 | 相对路径示例 | 完整访问地址 |
+|------|------------|------------|
+| coverImage | `/uploads/covers/1/abc.jpg` | `https://www.1415243231.top/uploads/covers/1/abc.jpg` |
+
+---
+
 ## 错误响应格式
 
 所有错误返回 `500 Internal Server Error`，响应体：