feat(docker): export/import (move a container to another machine) + boot reaper

- src/docker-export.ts: full-image export (pause-consistent commit + save|stream +
  workspace tar + manifest -> one .codeman-container.tgz) and workspace-only; import
  validates manifest + per-member sha256, traversal-guards the workspace tar, docker
  load + quarantine re-tag (never overwrites a local tag). Bounded by
  runWithConversionLimit; free-space precheck; docker rmi in finally; sealed
  containers refuse full-image export.
- routes: POST /api/docker-cases/:name/export (background + SSE), GET/DELETE
  /api/docker-exports, GET download, POST /api/docker-cases/import (-> new host+case)
- instance-scoped boot reaper (docker-hosts.reapOrphanedDockerContainers) wired after
  restoreMuxSessions; never touches another instance's containers
- SSE docker:exportComplete/exportFailed/importComplete (both registries)
- fix: stream pipeline in saveImageToTar so the bundle isn't truncated

VERIFIED end-to-end on real docker: full export -> 326MB valid bundle -> delete
case -> import -> new container runs from the quarantined image with the workspace
file AND the in-image change both restored.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/web/public/constants.js

@@ -474,6 +474,9 @@ const SSE_EVENTS = {
   CASE_LINKED: 'case:linked',
   CASE_DELETED: 'case:deleted',
   CASE_ORDER_CHANGED: 'case:order-changed',
+  DOCKER_EXPORT_COMPLETE: 'docker:exportComplete',
+  DOCKER_EXPORT_FAILED: 'docker:exportFailed',
+  DOCKER_IMPORT_COMPLETE: 'docker:importComplete',
 };
 
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