feat(notifications): replace hardcoded inbox notifications with subscriber-driven model

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>

server/pkg/protocol/events.go

@@ -39,6 +39,10 @@ const (
 	EventMemberUpdated = "member:updated"
 	EventMemberRemoved = "member:removed"
 
+	// Subscriber events
+	EventSubscriberAdded   = "subscriber:added"
+	EventSubscriberRemoved = "subscriber:removed"
+
 	// Daemon events
 	EventDaemonHeartbeat = "daemon:heartbeat"
 	EventDaemonRegister  = "daemon:register"