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/src/controllers/eventController.ts

@@ -23,7 +23,7 @@ async function findOrCreateUser(clerkId: string): Promise<string> {
     user = await prisma.user.create({
       data: {
         clerkId,
-        email: clerkUser.emailAddresses[0]?.emailAddress || `${clerkId}@temp.com`,
+        email: clerkUser.emailAddresses[0]?.emailAddress || null,
         username: clerkUser.username || `user_${clerkId.substring(0, 8)}`,
         firstName: clerkUser.firstName,
         lastName: clerkUser.lastName,