feat: implement email-optional authentication

- Make email field optional in database schema
- Update user creation to handle null emails
- Enhance notification service to gracefully skip email for users without email addresses
- Add validation in email service for null/empty emails
- Create comprehensive documentation for Clerk configuration
- Add test script for email-optional functionality
- Update all user creation points (userService, eventController, reportController)

Users can now sign up with just username and password. Email notifications
are gracefully skipped for users without email addresses, while in-app
notifications continue to work for all users.

Closes: Email-optional authentication requirement

server/prisma/migrations/20251118175855_make_email_optional/migration.sql

@@ -0,0 +1,3 @@
+-- AlterTable
+-- Make email field optional (nullable) in users table
+ALTER TABLE `users` MODIFY `email` VARCHAR(255) NULL;