fix: prevent duplicate user_ prefix in usernames

- Update user creation logic to strip 'user_' prefix from Clerk ID before generating username
- Change default username pattern from 'user_<clerkId>' to 'user<cleanId>'
- This prevents usernames like 'user_user_35Z...' and creates 'user35Z...' instead
- Apply fix to both userService and reportController
- Add migration script to fix existing duplicate usernames

Example: user_user_35ZFZ... -> user35ZFZ...

server/src/services/userService.ts

@@ -36,11 +36,15 @@ export class UserService {
     try {
       const clerkUser = await clerkClient.users.getUser(clerkId);
       
+      // Generate a clean username from Clerk ID (remove 'user_' prefix if present)
+      const cleanClerkId = clerkId.startsWith('user_') ? clerkId.substring(5) : clerkId;
+      const defaultUsername = clerkUser.username || `user${cleanClerkId.substring(0, 8)}`;
+      
       user = await prisma.user.create({
         data: {
           clerkId,
           email: clerkUser.emailAddresses[0]?.emailAddress || `${clerkId}@temp.com`,
-          username: clerkUser.username || `user_${clerkId.substring(0, 8)}`,
+          username: defaultUsername,
           firstName: clerkUser.firstName,
           lastName: clerkUser.lastName,
           avatarUrl: clerkUser.imageUrl,