v9.1.0

Changes

• Upgrade base image from python:3.9-slim-buster to python:3.13-slim-bookworm
• Sync all Python dependencies with alerta-server 9.1.0
• Add ARM64 multi-arch support (linux/amd64 + linux/arm64)
• Update MongoDB repo from 4.2 to 7.0, switch to mongosh
• Update Nginx repo to bookworm
• Make IPv6 nginx listen directive opt-in via ENABLE_IPV6 env var
• Unset SECRET_KEY env var after writing to config to fix precedence issue
• Don't exit on duplicate admin API key creation
• Bump GitHub Actions to fix Node.js 20 deprecation
• Use native ARM64 runners instead of QEMU emulation for faster builds