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>

apps/web/shared/api/client.ts

@@ -15,6 +15,7 @@ import type {
   DaemonPairingSession,
   ApproveDaemonPairingSessionRequest,
   InboxItem,
+  IssueSubscriber,
   Comment,
   Workspace,
   WorkspaceRepo,
@@ -200,6 +201,19 @@ export class ApiClient {
     await this.fetch(`/api/comments/${commentId}`, { method: "DELETE" });
   }
 
+  // Subscribers
+  async listIssueSubscribers(issueId: string): Promise<IssueSubscriber[]> {
+    return this.fetch(`/api/issues/${issueId}/subscribers`);
+  }
+
+  async subscribeToIssue(issueId: string): Promise<void> {
+    await this.fetch(`/api/issues/${issueId}/subscribe`, { method: "POST" });
+  }
+
+  async unsubscribeFromIssue(issueId: string): Promise<void> {
+    await this.fetch(`/api/issues/${issueId}/unsubscribe`, { method: "POST" });
+  }
+
   // Agents
   async listAgents(params?: { workspace_id?: string }): Promise<Agent[]> {
     const search = new URLSearchParams();