fix: Implement proper task deletion in cleanup button

- Add delete_task() method to DownloadManager that removes tasks from the task list
- Modify DELETE endpoint to use delete_task() instead of cancel_download()
- Tasks are now completely removed from the list when cleanup button is clicked

Previously, DELETE only cancelled the download but kept it in the list.
Now cancelled/failed/deleted downloads are permanently removed.

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via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

app/download_manager.py

@@ -75,6 +75,22 @@ class DownloadManager:
             if task.file_path and os.path.exists(task.file_path):
                 os.remove(task.file_path)
 
+    async def delete_task(self, task_id: str):
+        """Completely remove a task from the task list"""
+        task = self.tasks.get(task_id)
+        if task:
+            # Cancel if downloading
+            if task_id in self.active_downloads:
+                self.active_downloads[task_id].cancel()
+                del self.active_downloads[task_id]
+
+            # Delete file if exists
+            if task.file_path and os.path.exists(task.file_path):
+                os.remove(task.file_path)
+
+            # Remove from tasks dict
+            del self.tasks[task_id]
+
     async def _download(self, task: DownloadTask):
         async with self._semaphore:
             try: