build(version): inject real build version via ldflags (#47)

Dockerfile takes ARG VERSION (default "dev") and stamps it into
main.version with -X. CD passes --build-arg VERSION=<git tag on v* builds,
else the short sha>, so the running pod logs the actual build instead of
"dev". Verified locally: `-ldflags -X main.version=...` embeds the string.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/cd.yml

@@ -60,7 +60,11 @@ jobs:
         run: |
           REGISTRY="localhost:5000"
           REF="${REGISTRY}/${{ env.IMAGE }}:${{ steps.meta.outputs.sha-tag }}"
+          # Stamp the real build version: the git tag on a v* build, else the short sha.
+          VERSION="${{ steps.meta.outputs.version-tag }}"
+          [ -z "$VERSION" ] && VERSION="${{ steps.meta.outputs.sha-tag }}"
           buildah build \
+            --build-arg VERSION="${VERSION}" \
             --label "org.opencontainers.image.revision=${{ github.sha }}" \
             --label "org.opencontainers.image.source=${{ github.repositoryUrl }}" \
             -t ${REF} \