feat(backend): add operational smoke tooling and safe pricing cleanup endpoints

- add backend README and refresh API map and smoke regression docs
- add full backend smoke regression script
- add admin pricing cleanup preview and dry-run endpoints
- add helper script for test pricing cleanup
- verify typed error contracts, draft flow, publish readiness and preview flows
- verify publish preview retention and clean backend startup behavior

backend/docs/smoke-regression.md

@@ -17,6 +17,14 @@ export API_URL="http://127.0.0.1:9020"
 export API_KEY="admin-local-dev-key"
 export SCHEME_ID="82086336d385427f9d56244f9e1dd772"
 
+## Main script
+
+Primary operator regression:
+
+`backend/scripts/smoke_regression.sh`
+
+The script is expected to fail fast on any contract break or unexpected 5xx.
+
 ## 1. Health / system
 
 - GET /healthz -> 200
@@ -175,14 +183,14 @@ Validate:
 - GET /api/v1/admin/schemes/{scheme_id}/current/validation -> 200
 - GET /api/v1/admin/artifacts/publish-preview/audit -> 200
 - POST /api/v1/admin/artifacts/publish-preview/cleanup?dry_run=true -> 200
-
-Optional:
-- POST /api/v1/admin/schemes/{scheme_id}/current/display/regenerate?mode=passthrough -> 200
-- POST /api/v1/admin/display/backfill?mode=passthrough&limit=10&only_missing=true -> 200
+- GET /api/v1/admin/schemes/{scheme_id}/pricing/categories/cleanup-preview -> 200
+- POST /api/v1/admin/schemes/{scheme_id}/pricing/categories/cleanup with dry_run=true -> 200
 
 Validate:
-- audit endpoint does not report orphan files or missing files for DB rows in normal state
+- artifact audit does not report orphan files or missing files for DB rows in normal state
 - validation report is readable and deterministic
+- pricing cleanup preview returns matched candidates and safe_to_delete_count
+- pricing cleanup dry-run returns deleted_count=0 and would_delete_count>0
 - admin routes do not produce 500 for healthy scheme state
 
 ## 13. Audit trail
@@ -207,6 +215,7 @@ Regression is considered failed if any of the following happen:
 - draft summary / structure / validation / compare-preview returns 500
 - pricing bundle or diagnostics contract changes unexpectedly
 - admin audit/cleanup endpoints fail on healthy environment
+- pricing cleanup dry-run mutates data
 - artifact retention grows without bound for repeated preview refresh on same variant
 
 ## 15. Operator note
@@ -220,3 +229,4 @@ Run this checklist after:
 - startup/import/config changes
 - draft lifecycle changes
 - publish readiness changes
+- admin cleanup changes