Moved migrations into backend

backend/migrations/20260422094717_user_table.sql

@@ -0,0 +1,8 @@
+CREATE TABLE users (
+    id SMALLINT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
+    last_name VARCHAR(30),
+    first_name VARCHAR(30),
+    username VARCHAR(30),
+    pwd TEXT,
+    is_admin BOOLEAN NOT NULL DEFAULT false
+);