Fix user creation errors by handling empty emails properly

Adjust user creation schema to allow null emails, update data handling to convert empty strings to null, and provide a SQL script to clean up existing duplicate empty email entries, resolving the `users_email_key` constraint violation.

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Replit-Commit-Session-Id: a8a78c07-e900-425c-a577-5b4c5894379d
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fix-duplicate-empty-email.sql

@@ -0,0 +1,28 @@
+-- Fix duplicate empty email constraint issue in production
+-- This script converts empty string emails to NULL to allow multiple users without email
+
+-- Step 1: Update all empty string emails to NULL
+UPDATE users 
+SET email = NULL 
+WHERE email = '' OR email IS NULL OR TRIM(email) = '';
+
+-- Step 2: Verify the change
+SELECT 'Users with NULL email:' as status, COUNT(*) as count 
+FROM users 
+WHERE email IS NULL;
+
+-- Step 3: Verify no empty string emails remain
+SELECT 'Users with empty string email:' as status, COUNT(*) as count 
+FROM users 
+WHERE email = '';
+
+-- Step 4: Show sample of updated users
+SELECT id, username, 
+       CASE 
+         WHEN email IS NULL THEN '[NULL]' 
+         ELSE email 
+       END as email_display,
+       role 
+FROM users 
+ORDER BY username 
+LIMIT 10;