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).

i18n/en.sh

@@ -103,6 +103,8 @@ MSG[ml_prompt]="ML service URL (leave blank — backend degrades gracefully)"
 MSG[step_enrichment]="Metadata enrichment (optional)"
 MSG[enrichment_note]="Imported files are auto-tagged from their embedded tags. Add an AcoustID key to also identify untagged files by audio fingerprint (free: https://acoustid.org/new-application)."
 MSG[acoustid_prompt]="AcoustID API key (leave blank to use embedded tags only)"
+MSG[musicbrainz_note]="MusicBrainz/AcoustID require a contact email in their User-Agent or they may throttle requests."
+MSG[musicbrainz_email_prompt]="Contact email for MusicBrainz/AcoustID (leave blank to use the project's default)"
 
 # -- summary / run ---------------------------------------------------------
 MSG[summary_title]="Summary (secrets hidden)"