Configure GitHub auth/token so we can log in

[jefftriplett]

See #5

[jefftriplett]

Fighting "The redirect_uri is not associated with this application." but I pushed a settings change.

[jefftriplett]

GitHub OAuth login is now working end-to-end. Resolved across these commits:

• 37abcb9 — Honor X-Forwarded-Proto so allauth builds the https:// redirect_uri GitHub expects (fixes "The redirect_uri is not associated with this application").
• 86d6d3c — Enable SOCIALACCOUNT_EMAIL_AUTHENTICATION + AUTO_CONNECT so GitHub logins auto-link to existing User rows by verified email.
• 86b4f14 — Drop the staff-only social adapter; rely on the admin's built-in is_staff gate. New GitHub sign-ins create a non-staff User row; promote in /django-admin/auth/user/ to grant access.

Production env on Coolify must define GITHUB_CLIENT_ID and GITHUB_CLIENT_SECRET (the latter was the final blocker — settings reads GITHUB_CLIENT_SECRET).