Fix rescheduling recreated dynamic tasks

[saidM]

Fixes #792.

The scheduler reconciles dynamic tasks by key, so a task removed and recreated with the same key between polls currently looks unchanged.

This change tracks the database record associated with each scheduled dynamic task. When the same key points to a new record, the old timer is cancelled and the replacement is scheduled. It also prevents a callback from the previous task from restoring the old schedule.

Direct updates to an existing task record remain unchanged, as documented.

Testing

• Added coverage for replacing a dynamic task with the same key.
• Added coverage for direct updates remaining unchanged.
• Ran the full SQLite test suite: 357 tests, 1,744 assertions.
• Ran RuboCop.