Use repository variables for registry configuration

Match the working workflow pattern from other server by using
repository variables instead of hardcoded values:

- REGISTRY_HOST (code.puffinoffset.com)
- REGISTRY_USERNAME (matt)
- IMAGE_NAME (puffin-app)
- REGISTRY_TOKEN (secret with write:package permission)

This makes the workflow portable and matches the proven
working configuration.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.gitea/workflows/build-deploy.yml

@@ -18,9 +18,9 @@ jobs:
       - name: Log in to Gitea Container Registry
         uses: docker/login-action@v3
         with:
-          registry: code.puffinoffset.com
-          username: ${{ github.actor }}
-          password: ${{ secrets.GITHUB_TOKEN }}
+          registry: ${{ vars.REGISTRY_HOST }}
+          username: ${{ vars.REGISTRY_USERNAME }}
+          password: ${{ secrets.REGISTRY_TOKEN }}
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
@@ -32,5 +32,5 @@ jobs:
           platforms: linux/amd64
           push: true
           tags: |
-            code.puffinoffset.com/matt/puffin-app:latest
-            code.puffinoffset.com/matt/puffin-app:main-${{ github.sha }}
+            ${{ vars.REGISTRY_HOST }}/${{ vars.REGISTRY_USERNAME }}/${{ vars.IMAGE_NAME }}:latest
+            ${{ vars.REGISTRY_HOST }}/${{ vars.REGISTRY_USERNAME }}/${{ vars.IMAGE_NAME }}:main-${{ github.sha }}