Sign-in: bring an OIDC provider, never store passwords

Neon Law Navigator never stores a password — no password column, no hashing crate. Identity is delegated to an OIDC-compatible provider you bring, via the standard Authorization Code + PKCE flow. Four env vars wire it:

OAUTH_ISSUER_URL=...        # the provider's issuer; discovery hangs off /.well-known/openid-configuration
OAUTH_CLIENT_ID=...
OAUTH_CLIENT_SECRET=...
OAUTH_REDIRECT_URI=https://www.your-domain.example/auth/callback