fix: respect NIP-09 deletion events from relays

[barrydeen]

Summary

• Add kind 5 to engagement subscription filters so deletion events are fetched from relays
• Route kind 5 events through notification and engagement handlers in EventRouter
• Add author validation before honoring deletions — only delete events where the author matches, preventing malicious kind 5 events from removing other users' events

Test plan

• Publish a note, then delete it from another client — verify it disappears from the feed
• Verify that a kind 5 event from a different author does not remove someone else's note
• Confirm reactions, reposts, and zaps still work correctly with the updated filters