Patches the plane-backend GitHubOAuthProvider so the /auth/github/*
flow points at our self-hosted Zitadel instance when ZITADEL_DOMAIN
is set, and falls back to vanilla GitHub OAuth when unset (regression-
safe). Touch surface is one backend file plus a cosmetic frontend
label change. Full rationale, configuration steps, refresh procedure,
and AGPL compliance notes in BINARYBEACHIO.md at repo root.
