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.

templates/layout.html

@@ -65,6 +65,7 @@
     {{end}}
     <span class="slogan">We know good music, baby!</span>
     <span class="copyright">© Kessinen</span>
+    <span class="version" title="Käytössä oleva versio">v{{.Version}}</span>
   </footer>
 
   {{with .Flash}}