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.

在代码库中查找现有的任务路由，可能在 routes 或 controllers 目录中。PATCH 端点需要验证输入字段，更新数据库中的任务，并返回更新后的任务。检查模型或服务层中的任务更新逻辑和验证。确保响应符合指定的状态码。

由索引模型根据 Issue 内容生成。