Move auth onto the estate path

[ExtraToast]

Parent

JorisJonkers-dev/deploy-kit#195

What to build

Move the auth project (auth-api, auth-ui) onto the estate path:

• its project file, its Migration Proof written by CI, and its auth-migration image built FROM the platform runner;
• published on a release tag, composed, and handed over from fleet-infra;
• served blue-green behind the full Release Gate, with its migration run by the gate.

Acceptance criteria

• A tagged auth release migrates, switches blue-green across both members and promotes.
• Every forward-auth route keeps working through the cutover.
• fleet-infra no longer delivers auth.

Blocked by

• Slice 3b: render auth, with its migration Jobs and identity deploy-kit#200
• Release Gate: migrations, the guarded Down, held reporting and alerts delivery#3
• JorisJonkers-dev/homelab-collections#15