feat(search): raise NL search rate limit from 5 to 20 req/min

The rule-based NLP service is <100ms vs Ollama's ~15s, making the old
limit too restrictive for normal interactive use.

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

backend/src/main/resources/application.yaml

@@ -135,7 +135,7 @@ app:
 
   nl-search:
     rate-limit:
-      max-requests-per-minute: 5
+      max-requests-per-minute: 20
 
 ocr:
   sender-model: