feat(backend): add editor context, draft bootstrap flow and draft summary endpoints
- add editor context endpoint for published and draft state introspection - add ensure-draft endpoint to create or reuse the current editable draft - add draft summary endpoint with validation, diff and publish readiness summary - align draft read endpoints with optimistic concurrency via expected_scheme_version_id - fix startup, import and schema issues in editor entry flow - update smoke regression coverage for editor entry flow
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