feat: add user selection flow

backend/src/main/resources/db/migration/V1__create_users.sql

@@ -0,0 +1,7 @@
+CREATE TABLE app_user (
+    id UUID PRIMARY KEY,
+    name VARCHAR(50) NOT NULL,
+    normalized_name VARCHAR(150) NOT NULL,
+    created_at TIMESTAMP WITH TIME ZONE NOT NULL,
+    CONSTRAINT uk_app_user_normalized_name UNIQUE (normalized_name)
+);