Add guild-styled HTML templates for OIDC logout confirmation, post-logout
success, and error pages. Update wiki login heading to brand convention
(candlelight + warm-text). Restyle magic link email from blue to guild
colour tokens.
Add oidc-provider with MongoDB adapter so ghostguild.org can act as
the identity provider for the self-hosted Outline wiki. Members
authenticate via the existing magic-link flow, with automatic SSO
when an active session exists. Includes interaction routes, well-known
discovery endpoint, and login page.
