Add migration to add auth_user_id column to users table

- Introduced a new column 'auth_user_id' of type UUID to the 'users' table.
- Updated existing user data to set 'auth_user_id' to null for UserId 1.
- Created a unique index on 'auth_user_id' to enforce uniqueness.
- Implemented rollback functionality to remove the column and index if needed.

Backend/ERPCore/appsettings.Development.json

@@ -7,8 +7,5 @@
   },
   "ConnectionStrings": {
     "DefaultConnection": "Host=localhost;Port=5432;Database=ERPCore;Username=postgres;Password=root"
-  },
-  "Jwt": {
-    "SigningKey": "dev-only-signing-key-please-change-me-0123456789"
   }
 }