fix(aiot): snap端点支持图片直接返回，兼容前端<img>标签直接加载

前端将snap API URL直接设为<img src>，浏览器以Accept: image/*请求，
但原接口返回JSON导致图片无法显示。现通过检测Accept头自动判断：
- image/*请求：触发截图后直接返回JPEG字节流
- JSON请求：返回原有JSON格式

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/main/java/com/genersoft/iot/vmp/aiot/controller/AiRoiController.java

@@ -85,10 +85,26 @@ public class AiRoiController {
 
     @Operation(summary = "获取摄像头截图（Edge截图 → COS URL）")
     @GetMapping("/snap")
-    public Map<String, Object> getSnap(
+    public ResponseEntity<?> getSnap(
             @RequestParam String cameraCode,
-            @RequestParam(defaultValue = "false") boolean force) {
-        return screenshotService.requestScreenshot(cameraCode, force);
+            @RequestParam(defaultValue = "false") boolean force,
+            @RequestHeader(value = "Accept", defaultValue = "application/json") String accept) {
+
+        Map<String, Object> result = screenshotService.requestScreenshot(cameraCode, force);
+
+        // 如果是 <img> 标签发起的请求（Accept: image/*），直接返回图片字节
+        if (accept.contains("image/") && "ok".equals(result.get("status"))) {
+            byte[] image = screenshotService.proxyScreenshotImage(cameraCode);
+            if (image != null) {
+                return ResponseEntity.ok()
+                        .contentType(MediaType.IMAGE_JPEG)
+                        .header("Cache-Control", "max-age=60")
+                        .body(image);
+            }
+        }
+
+        // 默认返回 JSON（XHR/fetch 请求）
+        return ResponseEntity.ok(result);
     }
 
     @Operation(summary = "Edge 截图回调（Edge 主动调用）")