Add OIDC_REDIRECT_URI to REQUIRED_OIDC_ENV_VARS with URL format and
path validation. The redirect URI must be a parseable URL with a path
starting with /auth/callback. Localhost usage in production triggers
a warning but does not block startup.
This prevents 500 errors when BetterAuth attempts to construct the
authorization URL without a configured redirect URI.
Co-Authored-By: Claude Opus 4.6 <[email protected]>
