fixup: Refactor LSPS5 service: use LSPSDateTime, cleanups, fixes

- Use LSPSDateTime for webhook timestamps for future LDK serialization
- Make MIN_WEBHOOK_RETENTION_DAYS and PRUNE_STALE_WEBHOOKS_INTERVAL_DAYS Durations
- Remove unnecessary Arc from Mutex fields (webhooks, last_pruning)
- Use unwrap() for Mutex locks as per project convention
- Move pruning interval to a const
- Set last_pruning in prune_stale_webhooks after pruning
- Remove unnecessary reassignments and error handling on locks
- Use new_with_custom_time_provider for handler construction
- Set PROTOCOL_NUMBER to 5 for LSPS5
- Rename _new_with_custom_time_provider to new_with_custom_time_provider
- Make notification_cooldown_hours a Duration
- Minor doc and formatting improvements

Author: martinsaposnic