feat: Hub monitoring takeover — event push system + config cleanup (v0.21.0)
Replace external Healthchecks.io with Hub-native event system. Controller
now pushes structured events via POST /api/v1/event with typed detail
structs. Hub handles dead man's switch, notification dispatch, and cooldowns.
Phase 5: PushEvent() core method, 21 event types, expanded notification
settings (11 toggles), Hub connection monitoring on dashboard, alerts.
Phase 6: Deprecation log for ping UUIDs, pinger kept for transition.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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