feat: global failover + dashboard model config section

- Failover is now global (account-level, not per-channel)
- Dashboard shows role model config (Owner/Reviewer/Arbiter + MoA refs)
- Dashboard shows failover status when active
- Auto-clear failover on successful Claude rotation
- Remove per-channel lease writes from failover path

src/message-agent-executor.test.ts

@@ -53,6 +53,7 @@ vi.mock('./db.js', () => ({
 
 vi.mock('./service-routing.js', () => ({
   activateCodexFailover: vi.fn(),
+  clearGlobalFailover: vi.fn(),
   getEffectiveChannelLease: vi.fn(() => ({
     chat_jid: 'group@test',
     owner_service_id: 'claude',