Implement PATCH /tasks/:id

[0xironclad]

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.