Add multi-scrobbler for Plex now-playing scrobbling

Created by: jedmund

Summary

• Adds multi-scrobbler to the media-consumption stack to send Last.fm "now playing" updates from Plex (Plexamp only scrobbles after tracks finish)
• Adds scrobble.atelier.house DNS record and Traefik route with TinyAuth

Setup

Add these vault variables before deploying:

• multi_scrobbler_plex_token
• multi_scrobbler_lastfm_api_key
• multi_scrobbler_lastfm_api_secret

After deploy, visit scrobble.atelier.house and click "(Re)authenticate" on the Last.fm card to complete OAuth.

Test plan

• make deploy-media-consumption — container starts healthy
• make deploy-infra-gateway — DNS + Traefik route works
• Dashboard accessible behind TinyAuth at scrobble.atelier.house
• Last.fm OAuth completes successfully
• Play track in Plexamp → "now playing" shows on Last.fm profile