refactor: separate log timestamp from message as structured LogEntry

Introduced models.LogEntry{Message, CreatedAt} to replace raw strings
in the log pipeline. Timestamps are now formatted at render time, not
baked into stored messages.

- Engine: appendLog stores LogEntry with time.Now()
- Store: LoadLogs returns []LogEntry, selects created_at from DB
- Store: strips legacy [HH:MM] prefix from pre-refactor DB entries
- TUI: sidebar shows "MM/DD HH:MM" from CreatedAt
- TUI: full log view shows "MM/DD HH:MM" from CreatedAt
- SaveLog still receives plain message string (DB handles timestamp)

internal/store/storetest/mock.go

@@ -212,8 +212,8 @@ func (m *BaseMock) SaveLog(ctx context.Context, message string) error {
 	return nil
 }
 
-func (m *BaseMock) LoadLogs(_ context.Context, _ int) ([]string, error) { return nil, nil }
-func (m *BaseMock) PruneLogs(_ context.Context) error                   { return nil }
+func (m *BaseMock) LoadLogs(_ context.Context, _ int) ([]models.LogEntry, error) { return nil, nil }
+func (m *BaseMock) PruneLogs(_ context.Context) error                            { return nil }
 
 func (m *BaseMock) GetActiveMaintenanceWindows(ctx context.Context) ([]models.MaintenanceWindow, error) {
 	if m.GetActiveMaintenanceWindowsFunc != nil {