feat: log cmhub download diagnostics in GenerateWorker

GUI 半边配合 ai.py 的 cmhub 下载诊断：格式化「图片下载较慢」warning、
`cover_image_url` 调试行（脱敏 token、debug_only 不写入 SQLite run_log）；
`_log_run_event` 增加 persist 开关。tests/test_gui.py 覆盖脱敏与不持久化。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_gui.py

@@ -2008,6 +2008,27 @@ class GuiTests(TempDirMixin, unittest.TestCase):
                         "detail": "下载完成，1.3MB，耗时 12.4秒",
                     }
                 )
+                ai_cfg["on_event"](
+                    {
+                        "task": tasks[0],
+                        "phase": "cover",
+                        "step": "cover_download",
+                        "result": "warning",
+                        "detail": "图片下载较慢，已用 24.0秒，大小 1.3MB",
+                        "level": "warning",
+                    }
+                )
+                ai_cfg["on_event"](
+                    {
+                        "task": tasks[0],
+                        "phase": "cover",
+                        "step": "cover_image_url",
+                        "result": "debug",
+                        "detail": "cmhub 图片 URL：https://cdn.example.com/generated.png?token=SECRET-TOKEN",
+                        "level": "warning",
+                        "debug_only": True,
+                    }
+                )
                 ai_cfg["on_event"](
                     {
                         "task": tasks[0],
@@ -2046,6 +2067,8 @@ class GuiTests(TempDirMixin, unittest.TestCase):
             self.assertIn("准备重试 1/2", joined_logs)
             self.assertIn("cmhub 已返回 image_url，耗时 91.2秒", joined_logs)
             self.assertIn("下载完成，1.3MB，耗时 12.4秒", joined_logs)
+            self.assertIn("图片下载较慢，已用 24.0秒，大小 1.3MB", joined_logs)
+            self.assertIn("cmhub 图片 URL：https://cdn.example.com/generated.png?token=***", joined_logs)
             self.assertIn("本地保存完成，JPEG 已保存，耗时 1.1秒，文件 220.0KB", joined_logs)
             self.assertIn("token=***", joined_logs)
             self.assertNotIn("SECRET-TOKEN", joined_logs)
@@ -2055,6 +2078,8 @@ class GuiTests(TempDirMixin, unittest.TestCase):
             self.assertIn("[图片] 1/1 商品 51100639510", event_messages)
             self.assertIn("准备重试 1/2", event_messages)
             self.assertIn("下载完成，1.3MB，耗时 12.4秒", event_messages)
+            self.assertIn("图片下载较慢，已用 24.0秒，大小 1.3MB", event_messages)
+            self.assertNotIn("cmhub 图片 URL", event_messages)
 
         self.assert_removed(temp_dir)