refactor(search): remove NLP error codes and application config

Remove SMART_SEARCH_UNAVAILABLE and SMART_SEARCH_RATE_LIMITED error codes
from ErrorCode enum; remove nlp and nl-search configuration blocks from
application.yaml; remove nlp config block from application-dev.yaml.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/resources/application.yaml

@@ -130,13 +130,6 @@ app:
     # The loader maps columns by header name — no positional indices (see ADR-025).
     dir: ${IMPORT_DIR:/import}
 
-  nlp:
-    base-url: http://nlp-service:8001
-
-  nl-search:
-    rate-limit:
-      max-requests-per-minute: 20
-
 ocr:
   sender-model:
     activation-threshold: 100