Presence aggregation + deferred push suppression

[lhjt]

Summary

• Aggregate per-device presence into a single user presence state with refresh-window logic. This means that multiple devices reporting different presences will no longer cause the user's presence to flip frequently between updates.
• Make presence timers abortable and resilient to stale timer fires.
• Add in deferred push notifications, as is in other platforms like Discord. The current implementation is in-memory only to avoid having to making a schema change whilst this feature is being evaluated. Example:
  1. push notification is suppressed
  2. user goes unavailable/offline (no longer online)
  3. any unread messages that arrived and were suppressed will be pushed to the user as push notifications

Key changes

• Presence update pipeline extracted to src/service/presence/pipeline.rs
• New aggregation module src/service/presence/aggregate.rs and unit tests
• Presence timers now abort/reschedule safely; stale timers are ignored
• Deferred push suppression queue reintroduced (src/service/pusher/suppressed.rs) with enqueue/flush/clear hooks
• Presence inactivity now triggers suppressed push flush

Behaviour notes

• Refresh-skip now requires last_active_ago to be present; missing values will write a new event instead of skipping
• Aggregation returns Offline immediately when all devices are pruned

Tests

• cargo test -p tuwunel_service presence

[jevolk]

This is amazing work 😍

Thank you!!! 🙇🏻