Configurable user-agent via env var

src/main/resources/application.properties

@@ -11,7 +11,7 @@ quarkus.hibernate-orm.database.generation=update
 # MusicBrainz REST client (configKey="musicbrainz")
 quarkus.rest-client."musicbrainz".url=https://musicbrainz.org/ws/2
 # User-Agent (MBZ requires a meaningful contact string) - override per environment, never hardcode in code
-discdrop.mbz.user-agent=DiscDrop/1.0 ([email protected])
+discdrop.mbz.user-agent=${MBZ_USER_AGENT:DiscDrop/1.0 ([email protected])}
 discdrop.mbz.rate-limit-ms=1000
 discdrop.feed.page-size=25
 discdrop.rss.item-count=50