ci: add deploy workflow — retag SHA as latest on main merge

deploy.yaml (main push only):
  - Pull image by SHA tag (already pushed during PR CI)
  - Tag as :latest and push
  - No rebuild needed

.gitea/workflows/build.yaml

@@ -50,13 +50,19 @@ jobs:
           REGISTRY_USER: ${{ secrets.FORGEJO_REGISTRY_USER }}
           REGISTRY_TOKEN: ${{ secrets.FORGEJO_REGISTRY_TOKEN }}
 
-      - name: Build and push Docker image (SHA tag only)
+      - name: Build Docker image
         run: |
           docker build --no-cache --progress=plain \
             -t "${IMAGE}:${{ steps.sha.outputs.short_sha }}" \
+            -t "${IMAGE}:latest" \
             -f Dockerfile .
+
+      - name: Push Docker image
+        if: github.event_name == 'push' || github.event_name == 'workflow_dispatch'
+        run: |
           docker push "${IMAGE}:${{ steps.sha.outputs.short_sha }}"
-          echo "Pushed: ${IMAGE}:${{ steps.sha.outputs.short_sha }}"
+          docker push "${IMAGE}:latest"
+          echo "✓ Pushed: ${IMAGE}:${{ steps.sha.outputs.short_sha }}"
 
       - name: Prune unused images
         run: docker image prune -a --force 2>&1 | tail -3 || true