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

shared/schema.ts

@@ -460,6 +460,8 @@ export const insertUserSchema = createInsertSchema(users).pick({
   password: true,
   role: true,
   passwordChanged: true,
+}).extend({
+  email: z.union([z.string().email(), z.literal("")]).optional(),
 });
 
 export const insertGroupSchema = createInsertSchema(groups).omit({