Fix workflow checkpoint restore scoping

[3em0]

Summary

• require restore/update checkpoint requests to include the current session id
• scope checkpoint DAO reads and UI updates by both version id and session id
• scope workflow rewrite checkpoint reads to the active session
• rebuild checked-in frontend assets so restore/update calls send X-Session-ID

Fixes #149

Verification

• npm run build
• python -m compileall backend/dao/workflow_table.py backend/controller/conversation_api.py backend/service/workflow_rewrite_tools.py
• local end-to-end regression harness: Alice restore succeeds, Bob restore/update of Alice checkpoint fails
• git diff --check

Note

• npm run lint is currently blocked because the existing ESLint config references @typescript-eslint/eslint-plugin, which is not installed by npm ci.

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