Update .env.example

.env.example

@@ -1,6 +1,11 @@
 # Backend config
-DATABASE_URL=
+# Use a relative path so it works both locally and on Hugging Face
+DATABASE_URL=sqlite:///./backend/app.db
+
+# Security
 JWT_SECRET=your_super_secret_key_here
 JWT_EXPIRES_MINUTES=60
+
+# App settings
 APP_NAME=CHB App
 CORS_ORIGINS=*