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.

Look for existing task routes in the codebase, likely in a routes or controllers directory. The PATCH endpoint needs to validate the input fields, update the task in the database, and return the updated task. Check the model or service layer for task update logic and validation. Ensure the response matches the specified status codes.

索引モデルが issue の本文から書いたものです。