Restrict deployment to single server configuration

[row-of-code]

This PR removes multi-server deployment support to address several architectural limitations and edge cases that could lead to deployment failures:

• SSL certificate generation via HTTP challenge cannot be properly coordinated across multiple servers without additional orchestration
• Database replication and state management across distributed servers requires explicit configuration that's outside FTL's scope

Changes:

• Update server config in ftl.yaml to accept single server definition instead of array
• Remove server selection logic from cli commands
• Update documentation to reflect single-server architecture

The tool's primary goal of simplified deployments is better served by a focused single-server approach. For multi-server deployments, users should consider container orchestration platforms like Kubernetes.

[yarlson]

released