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/schema.prisma

@@ -9,7 +9,7 @@ datasource db {
 
 model User {
   id                 String              @id @default(dbgenerated("(uuid())")) @db.VarChar(36)
-  email              String              @unique(map: "email") @db.VarChar(255)
+  email              String?             @unique(map: "email") @db.VarChar(255)
   password_hash      String?             @db.VarChar(255)
   username           String              @unique(map: "username") @db.VarChar(100)
   createdAt          DateTime?           @default(now()) @map("created_at") @db.Timestamp(0)