feat(admin): compose service and image build

Adds admin to docker-compose.site.yml and a fourth Go image to the build
workflow, both following the conventions the other services already use.

LICENSE_SIGNING_KEY now appears in exactly one service in exactly one
compose file. It must never be added to server, and the self-hosted
docker-compose.yml still does not mention admin at all.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitea/workflows/server-deploy.yml

@@ -53,3 +53,10 @@ jobs:
                   # Root context: sitesvc depends on the shared module.
                   docker build -t "$IMAGE" -f sitesvc/Dockerfile .
                   docker push "$IMAGE"
+
+            - name: Build and push admin image
+              run: |
+                  IMAGE="${{ vars.DOCKER_HOST }}/${{ github.repository_owner }}/vantage/admin:latest"
+                  # Root context: admin depends on the shared module.
+                  docker build -t "$IMAGE" -f admin/Dockerfile .
+                  docker push "$IMAGE"