[dadofsambonzuki]

Summary

Recovers on-chain charge payments that currently only get detected via the mempool websocket push feed.

restart_address_tracking() only ran once at extension startup. If websocket tracking broke after that (for example once the number of tracked addresses hit mempool's per-connection limit and track-addresses started being rejected), on-chain payments were never detected until the next process restart. Each new charge then grew the rejected tracking list further, so the failure was permanent.

This PR converts that one-shot pass into a permanent periodic REST balance poll and hardens the settlement/webhook path:

• tasks.py – restart_address_tracking() now loops on a 120s cadence for the lifetime of the process, isolates per-charge errors, and drops paid and expired addresses from the websocket tracking list each pass so the list stays under mempool's per-connection address cap.
• helpers.py – check_charge_balance() now persists the settled charge before firing the webhook (mirroring tasks._handle_ws_message), so webhook handlers that re-fetch the charge (e.g. GET /satspay/api/v1/charge/{id}) observe it as paid. The webhook now fires only when a charge becomes paid, instead of on every balance check (which previously pinged the webhook endpoint on every poll of unpaid charges).
• init.py – registers the poll with create_permanent_unique_task (auto-restarts on failure) and includes it in scheduled_tasks so satspay_stop() cancels it.

Note

This is a stopgap for the public mempool.space feed. #88 (feat: use blockexplorer) supersedes this by moving on-chain tracking to a local block explorer.