Create demo accounts

DEMO.md

+# Demo
+There is a `db.demo.sqlite3` file containing demo data. You can use it to test the application.
+
+## Users
+- **Admin**
+  - Username: `demoadmin`
+  - Email: `demoadmin@example.com`
+  - Password: `DemoAdmin123!`
+- **User 1**
+  - Username: `demo1`
+  - Email: `demo1@example.com`
+  - Password: `DemoUser`
+  - Card number: `1111`
+- **User 2*
+  - Username: `demo2`
+  - Email: `demo2@example.com`
+  - Password: `DemoUser`
+  - Card number: `2222`
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