feat: show error reason when monitors go DOWN

Propagate check failure reasons through the entire stack:
- Checker captures specific errors (DNS, timeout, HTTP status, SSL, etc.)
- Engine tracks LastError, StatusChangedAt, LastSuccessAt per monitor
- State transitions persisted to new state_changes table
- Detail panel shows error reason, HTTP code, state duration, last
  success time, and last 5 state change events
- Monitor table shows inline error preview for DOWN services
- Alert messages include error reason
- Probe nodes forward error reasons to leader

15 files changed across models, checker, engine, store, TUI, and probes.

internal/cluster/cluster_test.go

@@ -61,13 +61,15 @@ func (m *mockStore) GetActiveMaintenanceWindows() ([]models.MaintenanceWindow, e
 func (m *mockStore) GetAllMaintenanceWindows(int) ([]models.MaintenanceWindow, error) {
 	return nil, nil
 }
-func (m *mockStore) AddMaintenanceWindow(models.MaintenanceWindow) error { return nil }
-func (m *mockStore) EndMaintenanceWindow(int) error                      { return nil }
-func (m *mockStore) DeleteMaintenanceWindow(int) error                   { return nil }
-func (m *mockStore) IsMonitorInMaintenance(int) (bool, error)            { return false, nil }
-func (m *mockStore) GetPreference(string) (string, error)                { return "", nil }
-func (m *mockStore) SetPreference(string, string) error                  { return nil }
-func (m *mockStore) Close() error                                        { return nil }
+func (m *mockStore) AddMaintenanceWindow(models.MaintenanceWindow) error    { return nil }
+func (m *mockStore) EndMaintenanceWindow(int) error                         { return nil }
+func (m *mockStore) DeleteMaintenanceWindow(int) error                      { return nil }
+func (m *mockStore) IsMonitorInMaintenance(int) (bool, error)               { return false, nil }
+func (m *mockStore) GetPreference(string) (string, error)                   { return "", nil }
+func (m *mockStore) SetPreference(string, string) error                     { return nil }
+func (m *mockStore) SaveStateChange(int, string, string, string) error      { return nil }
+func (m *mockStore) GetStateChanges(int, int) ([]models.StateChange, error) { return nil, nil }
+func (m *mockStore) Close() error                                           { return nil }
 
 // --- Cluster Start Tests ---