Unlock everything, add a donation link, credit Last.fm, nudge re-linking

- Every feature is free for everyone (the plan system stays for later);
  remaining limits are abuse guards
- Optional website donation link (NEXT_PUBLIC_DONATION_URL). Not shown in
  the mobile apps, where tips to the developer generally need in-app
  purchase
- Last.fm/ListenBrainz-sourced plays link to the user's profile, as
  Last.fm's API terms require
- Connections track when they last saw activity; a scrobbler silent for
  over a week prompts re-linking Spotify (its link expires every 6 months)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MzFywujuH9TiuXbr9vjoX8

docker-compose.yml

@@ -87,6 +87,8 @@ services:
       args:
         # The URL the *browser* uses to reach the API.
         NEXT_PUBLIC_API_URL: ${PUBLIC_API_URL:-http://localhost:4000}
+        # Optional donation link shown on the website, e.g. DONATION_URL=https://ko-fi.com/you
+        NEXT_PUBLIC_DONATION_URL: ${DONATION_URL:-}
     image: musicshare-web:local
     ports:
       - "${WEB_PORT:-3737}:3737"