OSS 结果文件下载分流：新增 aiimage.oss.download-endpoint 配置，result/ 前缀对象生成独立下载域名直链（默认未配置时回退原行为）

backend-java/src/test/java/com/nanri/aiimage/modules/file/service/oss/OssStorageServiceTest.java

@@ -12,10 +12,11 @@ import static org.junit.jupiter.api.Assertions.assertNull;
 class OssStorageServiceTest {
 
     private OssStorageService storageService;
+    private OssProperties properties;
 
     @BeforeEach
     void setUp() {
-        OssProperties properties = new OssProperties();
+        properties = new OssProperties();
         properties.setEndpoint("https://oss.aishufu.top");
         properties.setPublicEndpoint("https://oss.aishufu.top");
         properties.setBucket("nanri-ai-images");
@@ -93,4 +94,37 @@ class OssStorageServiceTest {
         assertNull(storageService.normalizeManagedPublicUrl(null));
         assertEquals("   ", storageService.normalizeManagedPublicUrl("   "));
     }
+
+    @Test
+    void resultFilesUseDownloadEndpointWhenConfigured() {
+        properties().setDownloadEndpoint("https://download.aishufu.top");
+        assertEquals(
+                "https://download.aishufu.top/nanri-ai-images/result/similar_asin/1/%E9%87%8D%E6%96%B0-result.xlsx",
+                storageService.generateFreshDownloadUrl("result/similar_asin/1/重新-result.xlsx"));
+        assertEquals(
+                "https://download.aishufu.top/nanri-ai-images/result/similar_asin/1/re.xlsx",
+                storageService.generateFreshDownloadUrl("https://oss.aishufu.top/nanri-ai-images/result/similar_asin/1/re.xlsx"));
+    }
+
+    @Test
+    void nonResultAndUnmanagedValuesKeepPublicEndpoint() {
+        properties().setDownloadEndpoint("https://download.aishufu.top");
+        assertEquals(
+                "https://oss.aishufu.top/nanri-ai-images/supply_images/main.jpg",
+                storageService.generateFreshDownloadUrl("supply_images/main.jpg"));
+        assertEquals(
+                "https://download.aishufu.top/shufu-video/result/image_video/demo.mp4",
+                storageService.generateFreshDownloadUrl("shufu-video/result/image_video/demo.mp4"));
+    }
+
+    @Test
+    void downloadEndpointDisabledFallsBackToPublicEndpoint() {
+        assertEquals(
+                "https://oss.aishufu.top/nanri-ai-images/result/similar_asin/1/re.xlsx",
+                storageService.generateFreshDownloadUrl("result/similar_asin/1/re.xlsx"));
+    }
+
+    private OssProperties properties() {
+        return properties;
+    }
 }