fix(gateway): Composite 分组放行视频生成端点

视频状态(/videos/:id)与内容下载(/videos/:id/content)都已放行
PlatformComposite, 唯独创建端点仅放行 PlatformGrok. Composite 分组
用户配好模型路由后可查询可下载, 唯独创建 404.

补齐放行判断, 与另外两个视频端点保持一致; 其他平台行为不变.

backend/internal/server/routes/gateway.go

@@ -93,7 +93,10 @@ func RegisterGatewayRoutes(
 		}
 	}
 	videoGenerationHandler := func(c *gin.Context) {
-		if getGroupPlatform(c) == service.PlatformGrok {
+		// Video status/content lookups below already allow Composite groups; keep
+		// task creation aligned so composite keys that route to Grok accounts can
+		// submit video generation jobs.
+		if platform := getGroupPlatform(c); platform == service.PlatformGrok || platform == service.PlatformComposite {
 			h.OpenAIGateway.GrokVideoGeneration(c)
 			return
 		}

backend/internal/server/routes/gateway_test.go

@@ -345,6 +345,18 @@ func TestGatewayRoutesNonGrokVideosAreRejectedAtPlatformGate(t *testing.T) {
 	}
 }
 
+func TestGatewayRoutesCompositeVideoGenerationAllowed(t *testing.T) {
+	router := newGatewayRoutesTestRouter(service.PlatformComposite)
+
+	req := httptest.NewRequest(http.MethodPost, "/v1/videos/generations", strings.NewReader(`{"model":"grok-imagine-video-1.5","prompt":"waves"}`))
+	req.Header.Set("Content-Type", "application/json")
+	w := httptest.NewRecorder()
+
+	router.ServeHTTP(w, req)
+	require.NotEqual(t, http.StatusNotFound, w.Code)
+	require.NotContains(t, w.Body.String(), "not supported")
+}
+
 func TestGatewayRoutesCompositeOpenAIOnlyEndpointsRequireOpenAITarget(t *testing.T) {
 	router := newGatewayRoutesTestRouter(service.PlatformComposite)