Make user creation and deletion more robust and prevent data loss

Update user schema to make username and password mandatory, adjust email validation, and handle foreign key constraints during user deletion, including updating the database_backups table.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: a8a78c07-e900-425c-a577-5b4c5894379d
Replit-Commit-Checkpoint-Type: full_checkpoint
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/1957c339-2757-4d1f-8e92-e9f71a1ce58e/a8a78c07-e900-425c-a577-5b4c5894379d/vetH0Gp

replit.md

@@ -90,7 +90,7 @@ Preferred communication style: Simple, everyday language.
   - Ownership transfer of all user-created records to existing admin before deletion
   - Comprehensive handling of all foreign key relationships including role assignments
   - Graceful error handling when no admin user exists
-- **Tables Handled**: customer_orders, orders, deliveries, publicities, dlcProducts, tasks, nocodbConfig, userGroups, userRoles (both userId and assignedBy)
+- **Tables Handled**: customer_orders, orders, deliveries, publicities, dlcProducts, tasks, nocodbConfig, userGroups, userRoles (both userId and assignedBy), database_backups
 - **Result**: Safe user deletion while preserving business data integrity
 
 #### User Name Fields Optional in Production