Fix review issues: detached contexts, loop device leak, timer leak, size_bytes

- Use context.Background() with timeout in destroySandbox/failBuild so
  cleanup and DB writes survive parent context cancellation on shutdown
- Fix loop device refcount leak in FlattenRootfs when dmDevice is nil
- Replace time.After with time.NewTimer in healthcheck polling to avoid
  goroutine leak when healthcheck passes early
- Capture size_bytes from CreateSnapshot/FlattenRootfs RPC responses
  instead of hardcoding 0 in the templates table insert
- Avoid leaking internal error details to API clients in build handler

internal/sandbox/manager.go

@@ -839,6 +839,8 @@ func (m *Manager) FlattenRootfs(ctx context.Context, sandboxID, name string) (in
 
 	outputPath := snapshot.RootfsPath(m.cfg.ImagesDir, name)
 	if sb.dmDevice == nil {
+		m.cleanupDM(sb)
+		warnErr("template dir cleanup error", name, snapshot.Remove(m.cfg.ImagesDir, name))
 		return 0, fmt.Errorf("sandbox %s has no dm device", sandboxID)
 	}