refactor: enforce enum machine codes

Use owner enums for error and T-Bank protocol values with ESLint regression checks.

Preserve open wire fallbacks and keep protocol imports independent of DI and Prisma.

Verified npm ci, verify, 226 unit tests, 262 isolated e2e tests and unchanged OpenAPI.

src/integrations/ncr/ncr-ratings.client.ts

@@ -4,6 +4,8 @@ import { LRUCache } from 'lru-cache';
 
 import { ISO_DATE_LENGTH, ISO_DATE_START } from '../../common/time';
 
+enum NcrErrorCode { Unavailable = 'NCR_UNAVAILABLE' }
+
 const EXPECTED_REGISTRY_TABLES = 1;
 const RATING_COLUMN = 2;
 
@@ -41,7 +43,7 @@ export class NcrRatingsClient {
         if (this.cache.get('registry') === pending) {
           this.cache.set('registry', pending, { ttl: 60_000 });
         }
-        throw new ServiceUnavailableException({ code: 'NCR_UNAVAILABLE' });
+        throw new ServiceUnavailableException({ code: NcrErrorCode.Unavailable });
       });
       this.cache.set('registry', pending);
     }