New environment variables:
- OIDC_ADMIN_CLAIM (default: "groups") — which claim to check
- OIDC_ADMIN_VALUE (e.g. "app-trek-admins") — value that grants admin
Admin role is resolved on every OIDC login:
- New users get admin if their claim matches
- Existing users have their role updated dynamically
- Removing a user from the group revokes admin on next login
- First user is always admin regardless of claims
- No config = previous behavior (first user admin, rest user)
Supports array claims (groups: ["a", "b"]) and string claims.
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