fix(auth): enforce single role per user

Change the user_role primary key from (user_id, role) to user_id alone, so each user holds at most one role row.

src/main/resources/db/migration/V7__user_role.sql

@@ -2,5 +2,5 @@ CREATE TABLE user_role
 (
     user_id BIGINT      NOT NULL REFERENCES app_user (id),
     role    VARCHAR(32) NOT NULL,
-    CONSTRAINT user_role_pkey PRIMARY KEY (user_id, role)
+    CONSTRAINT user_role_pkey PRIMARY KEY (user_id)
 );