regroup backend and frontend in app folder

app/backend/migrations/000002_auth_features.up.sql

@@ -0,0 +1,17 @@
+CREATE TABLE users (
+    id SERIAL PRIMARY KEY,
+    username VARCHAR(50) UNIQUE NOT NULL,
+    email VARCHAR(255) UNIQUE NOT NULL,
+    password_hash TEXT NOT NULL,
+    role VARCHAR(20) DEFAULT 'user',
+    created_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP,
+    last_login TIMESTAMP WITH TIME ZONE
+);
+
+CREATE TABLE sessions (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id INTEGER REFERENCES users(id) ON DELETE CASCADE,
+    token TEXT UNIQUE NOT NULL,
+    expires_at TIMESTAMP WITH TIME ZONE NOT NULL,
+    created_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP
+);