fix: 皮肤包删除后被自动迁移逻辑复活，IPA 下载连击刷屏操作日志

_migrate_old_themes 之前每次 load_config() 都会重跑，只要证书仍引用某皮肤
且旧版 themes 目录还在，删除后会被立即重新生成；现改为仅首次执行一次并
在 config.json 中打标记跳过后续调用。

打包历史下载 IPA 按钮此前无连击防护，前端加 in-flight 禁用态，后端对同一
用户同一任务的下载操作日志增加 5 秒去重，避免连击刷屏审计日志。

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/routers/tasks.py

@@ -3,6 +3,7 @@ import os
 import json
 import shutil
 import tempfile
+import time
 import uuid
 from datetime import datetime
 from typing import List
@@ -18,6 +19,10 @@ from ..deps import get_current_user
 
 router = APIRouter(prefix="/api/tasks", tags=["tasks"], dependencies=[Depends(get_current_user)])
 
+# 同一用户对同一任务连击下载按钮时，短时间内只记一次操作日志，避免刷屏
+_DOWNLOAD_LOG_DEBOUNCE_SECONDS = 5
+_recent_download_logs: dict[str, float] = {}
+
 
 @router.post("", response_model=TaskResponse)
 async def create_task(task: TaskCreate, request: Request, user: dict = Depends(get_current_user), db: Session = Depends(get_db)):
@@ -276,11 +281,22 @@ async def get_download_link(task_id: str, request: Request, user: dict = Depends
         )
     except (json.JSONDecodeError, DistributionError) as exc:
         raise HTTPException(status_code=502, detail=f"生成下载链接失败：{exc}") from exc
-    from ..services.audit_log import record_operation
-    record_operation(db, user, "ipa_download_requested", request=request, task=task,
-                     resource_type="ipa", resource_name=task.oss_url,
-                     detail={"controlled": True})
-    db.commit()
+
+    debounce_key = f"{user.get('username')}:{task_id}"
+    now = time.monotonic()
+    last = _recent_download_logs.get(debounce_key)
+    if last is None or now - last > _DOWNLOAD_LOG_DEBOUNCE_SECONDS:
+        if len(_recent_download_logs) > 500:
+            cutoff = now - _DOWNLOAD_LOG_DEBOUNCE_SECONDS
+            for key, ts in list(_recent_download_logs.items()):
+                if ts < cutoff:
+                    del _recent_download_logs[key]
+        _recent_download_logs[debounce_key] = now
+        from ..services.audit_log import record_operation
+        record_operation(db, user, "ipa_download_requested", request=request, task=task,
+                         resource_type="ipa", resource_name=task.oss_url,
+                         detail={"controlled": True})
+        db.commit()
     return {"url": download_url, "expires_in": DOWNLOAD_URL_EXPIRE_SECONDS}