Implement PATCH /tasks/:id

Implement a partial update endpoint for tasks.

What to implement

Route: PATCH /tasks/:id

Allow updating (any subset
```
name
description
run_at
priority
max_retries
status (e.g. pending, canceled)
```

Validate:
```
id is a valid number
run_at is a valid future datetime (for rescheduling)
status is one of the allowed values
```

Return:
```
200 OK with the updated task on success
404 if the task does not exist
400 on invalid input
```

Expected result
Clients can reschedule tasks, tweak metadata, or cancel them without recreating the task.

在程式碼庫中尋找現有的任務路由，可能在 routes 或 controllers 目錄中。PATCH 端點需要驗證輸入欄位，更新資料庫中的任務，並回傳更新後的任務。檢查模型或服務層中的任務更新邏輯和驗證。確保回應符合指定的狀態碼。

由索引模型根據 Issue 內容生成。