feat: Add staging environment setup

- Create staging stack with separate containers and volumes
- Configure staging.michaelschiemer.de subdomain routing
- Add deploy-staging.yml workflow for auto-deployment
- Extend build-image.yml to support staging branch
- Separate Redis instance and network for staging
- Staging uses staging branch by default

Features:
- Auto-deploy: Push to staging branch → build → deploy to staging
- Separate from production: Different containers, volumes, networks
- Shared Traefik: Uses same SSL certificates (*.michaelschiemer.de)
- Testing environment before production deployment

.gitea/workflows/build-image.yml

@@ -4,7 +4,7 @@ run-name: Build Image - ${{ github.ref_name }} - ${{ github.sha }}
 
 on:
   push:
-    branches: [ main, develop ]
+    branches: [ main, develop, staging ]
     paths-ignore:
       - '**.md'
       - 'docs/**'