feat: global Docker health indicator and graceful degradation

- GET /api/health endpoint returning Docker connectivity status
- Sidebar shows Docker connection dot (green=connected, red=disconnected)
- Stale scanner returns store-only results when Docker is unavailable
- Polls health every 30s

internal/stale/scanner.go

@@ -216,7 +216,9 @@ func (s *Scanner) FindStaleInstances(ctx context.Context) ([]StaleInstance, erro
 
 	containers, err := s.docker.ListContainers(ctx, nil)
 	if err != nil {
-		return nil, fmt.Errorf("list docker containers: %w", err)
+		// Docker unavailable — fall back to store-only detection (no live state).
+		slog.Warn("stale scanner: docker unavailable, using store status only", "error", err)
+		containers = nil
 	}
 
 	containerStateByInstanceID := make(map[string]string, len(containers))