Add CalVer versioning and rebuild the song page's fact line

Versions are YYYY.MM.DD-N, injected with -ldflags from a git tag, so no file
in the repo carries the number and a local build honestly says dev. The
version shows in the footer, the startup log and /healthz.

The song page's metadata was five different kinds of fact — artist, genre,
duration, provenance and a badge about the viewer — in one flat run with two
competing pills. Now the artist has its own line in the display face, and the
facts sit in four labelled cells like the spine of a cassette insert. The
submitter links to their profile, "oma kappale" became "lähetti: sinä", and
the clock time is gone: nobody needs the minute a song was published.

docker-compose.yml

@@ -17,7 +17,10 @@ services:
     restart: unless-stopped
 
   app:
-    build: .
+    build:
+      context: .
+      args:
+        VERSION: ${VERSION:-dev}
     environment:
       DATABASE_URL: postgres://levyraati:${POSTGRES_PASSWORD}@postgres:5432/levyraati
       ADMIN_USER: ${ADMIN_USER:-admin}