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/db/queries/subscriber.sql

@@ -0,0 +1,19 @@
+-- name: AddIssueSubscriber :exec
+INSERT INTO issue_subscriber (issue_id, user_type, user_id, reason)
+VALUES ($1, $2, $3, $4)
+ON CONFLICT (issue_id, user_type, user_id) DO NOTHING;
+
+-- name: RemoveIssueSubscriber :exec
+DELETE FROM issue_subscriber
+WHERE issue_id = $1 AND user_type = $2 AND user_id = $3;
+
+-- name: ListIssueSubscribers :many
+SELECT * FROM issue_subscriber
+WHERE issue_id = $1
+ORDER BY created_at;
+
+-- name: IsIssueSubscriber :one
+SELECT EXISTS(
+    SELECT 1 FROM issue_subscriber
+    WHERE issue_id = $1 AND user_type = $2 AND user_id = $3
+) AS subscribed;