feat(wizard): prompt for MusicBrainz/AcoustID contact email

Replace the hardcoded MUSICBRAINZ_USER_AGENT placeholder in env.template
with an optional MUSICBRAINZ_OWNER_EMAIL prompt in step_enrichment.
The backend now composes a valid User-Agent from app name + version +
this email (falling back to the project URL if left blank).

templates/env/env.template

@@ -33,7 +33,6 @@ STORAGE_BACKEND=@STORAGE_BACKEND@
 # Browser-facing API base URL. Injected into the webui at container start
 # (window.__APP_CONFIG__). '/api/v1' = same-origin behind the reverse proxy.
 PUBLIC_API_BASE_URL=@PUBLIC_API_BASE_URL@
-MUSICBRAINZ_USER_AGENT=mcma-backend/0.1.0 ( https://github.com/your/repo )
 
 # -- published ports ------------------------------------------------------
 HTTP_PORT=@HTTP_PORT@