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.

Dockerfile

@@ -1,9 +1,11 @@
 FROM golang:1.26-alpine AS build
+# CalVer, injected at build so no file needs bumping by hand: docker build --build-arg VERSION=…
+ARG VERSION=dev
 WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download
 COPY . .
-RUN CGO_ENABLED=0 go build -o /levyraati .
+RUN CGO_ENABLED=0 go build -ldflags "-X main.version=${VERSION}" -o /levyraati .
 
 FROM alpine:3.24
 # yt-dlp rots against YouTube. Alpine's active branch tracks it closely (3.24 carries the current