feat(radio): add configurable priority-country coverage

.env.example

@@ -248,3 +248,8 @@ OVERPASS_UPSTREAMS=
 # frame-ancestors sources such as "https://example.com", or * for any page.
 # Unset, no page may frame it. Other pages of the app can never be framed.
 # GEV_EMBED_FRAME_ANCESTORS=
+
+# Optional keyless Radio Browser coverage: up to 8 ISO alpha-2 countries.
+# Each reserves up to 40 stations within the existing 750-station directory.
+# Restart after changing. Leave blank for the existing global selection.
+# RADIO_PRIORITY_COUNTRIES=IN,PL