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.

main.go

@@ -3,6 +3,7 @@ package main
 import (
 	"context"
 	"crypto/subtle"
+	"fmt"
 	"log/slog"
 	"net/http"
 	"os"
@@ -13,6 +14,9 @@ import (
 	"github.com/jackc/pgx/v5/pgxpool"
 )
 
+// Set at build time with -ldflags "-X main.version=…". A local `go build` honestly says dev.
+var version = "dev"
+
 type config struct {
 	databaseURL   string
 	adminUser     string
@@ -67,6 +71,7 @@ type app struct {
 
 func main() {
 	slog.SetDefault(slog.New(slog.NewJSONHandler(os.Stdout, nil)))
+	slog.Info("starting", "ctx", "startup", "version", version)
 	cfg := loadConfig()
 
 	ctx := context.Background()
@@ -126,7 +131,8 @@ func (a *app) memberMux() *http.ServeMux {
 			http.Error(w, "db down", http.StatusServiceUnavailable)
 			return
 		}
-		w.Write([]byte("ok"))
+		// The version answers "what is actually running out there" without an SSH session.
+		fmt.Fprintf(w, "ok %s\n", version)
 	})
 
 	mux.HandleFunc("GET /login", a.loginPage)