Add Cloudflare quick tunnel support for manager URLs

[hermabr]

Summary

• Adds a pluggable manager connection abstraction with direct and Cloudflare quick tunnel implementations.
• Routes manager startup through an advertised URL while keeping a local loopback origin for tunnel setup.
• Updates Slurm backend defaults to use Cloudflare quick tunnels when no worker connect host is configured.
• Adds tests for tunnel startup, timeout/exit handling, URL rewriting, and manager connection selection.

Testing

• Not run
• Covered by new tests for:
  • direct connection passthrough
  • cloudflared command validation and PATH errors
  • trycloudflare URL parsing from subprocess output
  • timeout and early-exit cleanup behavior
  • Slurm backend URL rewriting and defaults
  • manager/server advertised URL selection and conflict detection