Add release->prod deploy (per-service, PROD_SSH_* secrets)

Claude-Session: https://claude.ai/code/session_01RdARHDJaYC1wSQRTYM6t3Z

.forgejo/workflows/deploy-prod.yml

@@ -0,0 +1,55 @@
+name: Deploy backend to prod VPS
+
+# On a push to `release`, SSH to prod and roll ONLY the backend service onto
+# the image build-push.yml just published for this commit. This is the
+# release->prod path, distinct from deploy.yml's main->beta path -- same
+# shape, different branch, different target host, and a completely separate
+# secret set (PROD_SSH_HOST/USER/KEY/PORT), so a beta credential leak can't
+# touch prod and vice versa. Frontend is deployed independently by its own
+# repo's deploy-prod.yml -- that is the whole point of the FE/BE CI-CD split:
+# a backend change ships without touching frontend and vice versa.
+# thermograph-infra/deploy/deploy.sh persists each service's live tag
+# host-side, so a single-service roll never disturbs the other.
+#
+# The prod SSH user (agent) has full sudo because the SOPS secrets render
+# sudo-reads the age key on that box, but that's handled inside deploy.sh --
+# nothing extra is needed here. appleboy/ssh-action is referenced by full
+# GitHub URL because it isn't mirrored in Forgejo's default action registry.
+#
+# The tag MUST match build-push.yml's `sha-${GITHUB_SHA:0:12}` (12 hex), not
+# the full 40-char ${{ github.sha }} -- default Actions expressions have no
+# substring function, so the compute step below truncates it. deploy.sh also
+# retries the pull for ~5 min because Forgejo `needs:` can't gate across the
+# separate build-push.yml, so this push's build may still be in flight.
+
+on:
+  push:
+    branches: [release]
+  workflow_dispatch: {}
+
+concurrency:
+  group: prod-deploy-backend
+  cancel-in-progress: false
+
+jobs:
+  deploy:
+    runs-on: docker
+    steps:
+      - name: Compute image tag
+        id: tag
+        run: echo "tag=sha-${GITHUB_SHA:0:12}" >> "$GITHUB_OUTPUT"
+
+      - name: Deploy backend over SSH
+        uses: https://github.com/appleboy/ssh-action@v1.2.0
+        with:
+          host: ${{ secrets.PROD_SSH_HOST }}
+          username: ${{ secrets.PROD_SSH_USER }}
+          key: ${{ secrets.PROD_SSH_KEY }}
+          port: ${{ secrets.PROD_SSH_PORT }}
+          # Forward the target service + the image tag to run. deploy.sh reads
+          # BACKEND_IMAGE_TAG and rolls just `backend`.
+          envs: SERVICE,BACKEND_IMAGE_TAG
+          script: /opt/thermograph/deploy/deploy.sh
+        env:
+          SERVICE: backend
+          BACKEND_IMAGE_TAG: ${{ steps.tag.outputs.tag }}