Backend Release v0.57.0

This PR prepares a new backend release.

When merged, this will:

1. Create a release tag
2. Build and publish a multi-architecture Docker image to Docker Hub

The Docker image will be available at:

• deploystack/backend:latest
• deploystack/backend:v0.57.0

Supported Architectures

• linux/amd64 (Intel/AMD)
• linux/arm64 (Apple Silicon, AWS Graviton)

Release notes:

• style(backend): improve email template styles and layout (290b7d84)
• fix(backend): log database status during authentication and server startup (31e5497b)
• feat(backend): add email notifications for global admin role changes (843f51aa)
• chore(all): update deploystack banner image (924cbcb5)
• feat(all): implement per-user MCP instance isolation for multi-user teams (bc9d1833)
• refactor(all): move admin user routes to /api/admin/users (c319dd06)