Split Docker Compose into base/dev/prod/traefik overrides with multi-instance support

- Split docker-compose.yml into base config + environment overrides
  - docker-compose.dev.yml: port mappings for local development
  - docker-compose.prod.yml: external proxy-net network, no exposed ports
  - docker-compose.traefik.yml: optional Traefik auto-discovery labels
- Parameterize container and volume names with COMPOSE_PROJECT_NAME
  - Default: hub-dev (containers: hub-dev-api, volumes: hub-dev_hub_data)
  - Override per instance for multi-instance deployments (hub-prod, hub-beta)
- Add Makefile with build/up/down/logs/backup/restore targets
- Add TRAEFIK_DOMAIN env var for Traefik routing configuration
- Update UPGRADING.md with volume migration instructions (rename + copy methods)
- Update README.md with multi-instance deployment and backup/restore sections

.gitignore

@@ -3,6 +3,7 @@
 !example/data/
 /seed/
 !example/seed/
+/backup/
 /content/
 
 # Byte-compiled / optimized / DLL files