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.

docs/decisions.md

@@ -185,3 +185,9 @@ says so.
     changing under a future client — and the first endpoint gets built the day something actually
     calls it. Both surfaces being thin adapters over one data function is already true of the page
     handlers, so adding the JSON side later stays a one-line-per-route job.
+44. **CalVer, `YYYY.MM.DD-N`, injected at build time.** The date is the useful part: this app ships
+    when there is something to ship and gets rebuilt monthly for yt-dlp anyway, so a semantic
+    version would communicate nothing a date does not. `N` is the build number within that day,
+    starting at 1, for the second attempt at a release. The string lives in a git tag and reaches
+    the binary through `-ldflags`, so no file in the repo has to be bumped and a local build
+    honestly reports `dev`. It surfaces in the footer, the startup log and `/healthz`.