docs: add DATABASE_URL to .env.example (#172)

- Add DATABASE_URL example to .env.example for new installations
- Ensures new users have the required Prisma database URL configured

Author: michelroegl-brunner

.env.example

@@ -25,4 +25,5 @@ AUTH_USERNAME=
 AUTH_PASSWORD_HASH= 
 AUTH_ENABLED=false
 AUTH_SETUP_COMPLETED=false
-JWT_SECRET=
+JWT_SECRET=
+DATABASE_URL="file:./data/database.sqlite"