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/song.html

@@ -1,13 +1,18 @@
 {{define "content"}}
 {{$s := .Data}}
 <h1>{{$s.Title}}</h1>
-<p class="songmeta">
-  <strong>{{$s.Artist}}</strong>
-  <span class="badge genre">{{$s.GenreLabel}}</span>
-  <span>{{$s.Length}}</span>
-  <span>lähetti {{$s.Submitter}} {{fidate $s.CreatedAt}}</span>
-  {{if $s.Own}}<span class="badge admin">oma kappale</span>{{end}}
-</p>
+<p class="by">{{$s.Artist}}</p>
+
+<!-- Four different kinds of fact, so four labelled cells rather than one run of text. "Oma
+     kappale" belongs here as the submitter, not as a badge: it is a fact about you. -->
+<dl class="spec">
+  <div><dt>Genre</dt><dd>{{$s.GenreLabel}}</dd></div>
+  <div><dt>Kesto</dt><dd>{{$s.Length}}</dd></div>
+  <div><dt>Lähetti</dt>
+    <dd>{{if $s.Own}}sinä{{else}}<a href="/profile/{{$s.SubmitterID}}">{{$s.Submitter}}</a>{{end}}</dd>
+  </div>
+  <div><dt>Julkaistu</dt><dd>{{fiday $s.CreatedAt}}</dd></div>
+</dl>
 
 {{if $s.CanEdit}}
 <details class="editbox">