- Extend what was `NotificationWorkerFactory` to `WorkerFactory`. This
can construct arbitrary Workers as long as they provide their own
Factory for construction.
The per-Worker factory contains any injected components just for that
worker type, keeping `WorkerFactory` clean.
- Move `NotificationWorkerFactory` to the new model.
- Implement `PruneCacheWorker`, and remove the code from
`CachedTimelineViewModel`.
- Create the periodic worker in `TuskyApplication`, ensuring that the
database is only pruned when the device is idle.
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