feat: notify new payment requests while bitkit is in the background

[ovitrif]

Twin: synonymdev/bitkit-ios#808
Refs: #1340

Problem or use case

Bitkit finds Paykit payment requests only while it is open: requests are discovered by polling the contact's homeserver, and no push exists for them. An allower who is not in the app never learns that a request is waiting, whether an allowance covers it or not.

Proposed solution

First version of background support for allowances: poll for new Paykit requests in the background and notify. No payment happens in the background.

• One local notification per new request, such as "{contact} requests $2 · Open Bitkit to pay it". For a request an allowance covers, the text says it is paid automatically when Bitkit opens.
• Each request notifies once. Tapping the notification opens Bitkit on that request's Payment Request sheet.
• Nothing is posted when notifications are not allowed.

Platform details (Android):

• A periodic CoroutineWorker on WorkManager with a connected-network constraint, scheduled like AppWidgetRefreshScheduler. WorkManager runs it at most every 15 minutes, and Doze can defer it further.
• The worker refreshes Paykit only: it does not start the Lightning node, so it needs no foreground service.
• It uses the same Hilt singletons as the app, so PaykitSdkService's mutexes serialize it against a refresh from the foreground app.
• The allowance coverage check moves out of AppViewModel, where it only runs while the UI is alive, into a repository the worker can call.
• Notifications go through the existing pushNotification helper, only when POST_NOTIFICATIONS is granted. The IDs of notified requests are persisted so a request notifies once, including across process death.

Done when:

• A request sent while Bitkit is backgrounded or closed produces exactly one notification within the platform's polling window.
• Opening Bitkit from that notification shows the request, or pays it when an allowance covers it.
• A request that was already notified, or already handled in the app, does not notify again.

Alternatives considered

• A server-side push when a request lands, for example from Paykit Server or the Blocktank notifier watching the homeserver. It would notify within seconds, but it needs backend and protocol work; tracked separately.
• Paying covered requests in the background. Left for a follow-up.

Additional context

• Builds on the implementation in the refs above.