feat(idp): add keycloak-first support with authentik fallback

backend/scripts/init_schema.sql

@@ -20,7 +20,7 @@ DROP TABLE IF EXISTS permissions CASCADE;
 CREATE TABLE users (
   id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
   user_sub TEXT NOT NULL UNIQUE,
-  idp_user_id INTEGER,
+  idp_user_id VARCHAR(128),
   username TEXT UNIQUE,
   email TEXT UNIQUE,
   display_name TEXT,