feat(backend): stabilize draft editor flow and complete smoke regression baseline

- add editor entry flow with editor context and ensure-draft bootstrap
- add draft summary read model and single-record draft read endpoints
- add typed draft, edit and publish conflicts with validation errors
- add pricing diagnostics and publish readiness endpoints
- fix Decimal serialization in seat price and test preview flows
- harden draft lifecycle guards for published vs draft current version
- update API map and smoke regression checklist
- add backend README and smoke regression script

backend/docs/api-map.md

@@ -21,6 +21,7 @@
 - GET /api/v1/schemes/{scheme_id}/versions
 - POST /api/v1/schemes/{scheme_id}/versions
 - GET /api/v1/schemes/{scheme_id}/publish/validation
+- GET /api/v1/schemes/{scheme_id}/draft/publish-readiness
 - POST /api/v1/schemes/{scheme_id}/publish
 - POST /api/v1/schemes/{scheme_id}/unpublish
 - POST /api/v1/schemes/{scheme_id}/rollback
@@ -58,7 +59,6 @@
 ## app/api/routes/publish.py
 - POST /api/v1/schemes/{scheme_id}/draft/pricing/snapshot
 - GET /api/v1/schemes/{scheme_id}/draft/publish-preview
-- GET /api/v1/schemes/{scheme_id}/draft/publish-readiness
 - POST /api/v1/schemes/{scheme_id}/draft/remap/preview
 - POST /api/v1/schemes/{scheme_id}/draft/remap/apply
 
@@ -94,4 +94,4 @@
 - This file is an operational route index, not a generated OpenAPI export.
 - Update this map in the same change set when adding, removing, renaming, or moving routes.
 - Query guards such as expected_current_scheme_version_id / expected_scheme_version_id are part of the operational contract for optimistic concurrency on mutable flows.
-- Draft/editor routes may legally return 409 draft_not_editable when current version is already published and no editable draft exists yet.
+- Draft editor flow starts from editor/context and draft/ensure, not from direct blind mutation calls.