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>

app/gui/workers.py

@@ -159,7 +159,12 @@ class GenerateWorker(BaseWorker):
         message = self._format_generation_event(payload)
         if not message:
             return
-        self._log_run_event(message, task=task, level=payload.get("level") or "info")
+        self._log_run_event(
+            message,
+            task=task,
+            level=payload.get("level") or "info",
+            persist=not bool(payload.get("debug_only")),
+        )
 
     def _remember_cmhub_metadata(self, payload):
         metadata = payload.get("metadata")
@@ -195,10 +200,15 @@ class GenerateWorker(BaseWorker):
                 return f"[停止] {self._task_plain_label(task)} 标题生成已取消"
             return None
         if phase == "cover":
+            if result == "debug" and step == "cover_image_url":
+                debug_detail = self._debug_detail(payload.get("detail"))
+                return f"[调试] {self._task_progress_label(task)} {debug_detail}"
             if result == "start" and step == "cover_submit":
                 return f"[图片] {self._task_progress_label(task)} 开始生成"
             if result == "success" and step == "cover_request":
                 return f"[图片] {self._task_progress_label(task)} {detail or 'cmhub 已返回，耗时未知'}"
+            if result == "warning" and step == "cover_download":
+                return f"[图片] {self._task_progress_label(task)} {detail or '图片下载较慢'}"
             if result == "success" and step == "cover_download":
                 return f"[图片] {self._task_progress_label(task)} {detail or '图片下载完成，耗时未知'}"
             if result == "success" and step == "cover_save":
@@ -267,6 +277,14 @@ class GenerateWorker(BaseWorker):
             return text[:177] + "..."
         return text
 
+    def _debug_detail(self, detail):
+        if detail is None:
+            return ""
+        text = diagnostics.redact_log_text(str(detail)).replace("\r", " ").replace("\n", " ").strip()
+        if len(text) > 2000:
+            return text[:1997] + "..."
+        return text
+
     def _format_generate_completion(self, summary):
         progress = self._summary_text(summary)
         billing_error = summary.get("billing_error") or {}
@@ -382,10 +400,10 @@ class GenerateWorker(BaseWorker):
         except Exception:
             return
 
-    def _log_run_event(self, message, task=None, level="info"):
+    def _log_run_event(self, message, task=None, level="info", persist=True):
         safe_message = diagnostics.redact_log_text(message)
         self.log.emit(str(safe_message))
-        if self._run_id is None:
+        if self._run_id is None or not persist:
             return
         try:
             db.add_run_log_event(