Add rails encryption for totp - allow configuration of encryption secrets from env, or derive them from SECRET_KEY_BASE. Don't leak email address via web_authn, rate limit web_authn, escape oidc state value, require password for changing email address, allow settings the hmac secret for token prefix generation

.env.example

@@ -1,5 +1,21 @@
 # Rails Configuration
-SECRET_KEY_BASE=generate-with-bin-rails-secret
+# SECRET_KEY_BASE is used for:
+#   - Session cookie encryption
+#   - Signed token verification
+#   - ActiveRecord encryption (currently: TOTP secrets)
+#   - OIDC token prefix HMAC derivation
+#
+# CRITICAL: Do NOT change SECRET_KEY_BASE after deployment. Changing it will:
+#   - Invalidate all user sessions (users must re-login)
+#   - Break encrypted data (users must re-setup 2FA)
+#   - Invalidate all OIDC access/refresh tokens (clients must re-authenticate)
+#
+# Optional: Override encryption keys with env vars for key rotation:
+#   - ACTIVE_RECORD_ENCRYPTION_PRIMARY_KEY
+#   - ACTIVE_RECORD_ENCRYPTION_DETERMINISTIC_KEY
+#   - ACTIVE_RECORD_ENCRYPTION_KEY_DERIVATION_SALT
+#   - OIDC_TOKEN_PREFIX_HMAC
+SECRET_KEY_BASE=generate-with-bin/rails/secret
 RAILS_ENV=development
 
 # Database