regroup backend and frontend in app folder

app/backend/migrations/000003_api_keys.up.sql

@@ -0,0 +1,7 @@
+CREATE TABLE node_api_keys (
+    id SERIAL PRIMARY KEY,
+    key_name VARCHAR(100),
+    key_value VARCHAR(64) UNIQUE NOT NULL,
+    is_active BOOLEAN DEFAULT true,
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);