Replace inbox_listeners.go with a subscriber-driven notification system:
- Add issue_subscriber table with auto-subscribe on create/assign/comment
- New subscriber_listeners.go: maintains subscriber data on domain events
- New notification_listeners.go: notifySubscribers (fanout to all subscribers
minus actor) and notifyDirect (targeted, punches through unsubscribe)
- Subscriber API: list/subscribe/unsubscribe endpoints
- Frontend: subscribers section in issue detail sidebar with real-time sync
- Frontend: inbox notification grouping by (issue_id, type, actor_id)
- Remove createInboxForIssueCreator from task.go (unified through event bus)
- 21 new Go tests, all passing
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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