docs(env): add configuration for country service cache duration

- Add new optional environment variable COUNTRY_SERVICE_CACHE_MINUTES
- Specifies the cache duration for the CountryQueryService in minutes
- Defaults to 15 minutes if not specified

Author: fulleni

.env.example

@@ -52,3 +52,7 @@
 
 # OPTIONAL: Window for the /data API endpoints, in minutes.
 # RATE_LIMIT_DATA_API_WINDOW_MINUTES=60
+
+# OPTIONAL: The cache duration for the CountryQueryService, in minutes.
+# Defaults to 15 minutes if not specified.
+# COUNTRY_SERVICE_CACHE_MINUTES=15