fix(email): include course name in student welcome subject (#739)

* fix(email): include course name in student welcome subject

The studentCourseWelcome template used a static subject, so bulk course
assignments produced identical inbox titles. Resolve the subject from
courseName at send time and sanitize it like other email headers.

Co-authored-by: Rotimi Best <rotimi-best@users.noreply.github.com>

* fix(email): use course-only subject for student welcome email

Co-authored-by: Rotimi Best <rotimi-best@users.noreply.github.com>

---------

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Rotimi Best <rotimi-best@users.noreply.github.com>

packages/email/src/emails/student-course-welcome.ts

@@ -6,7 +6,7 @@ import { ZEmailBranding } from '../core/branding';
 
 export const studentCourseWelcomeEmail = defineEmail({
   id: 'studentCourseWelcome',
-  subject: 'You have access to a course — sign in to get started',
+  subject: (fields) => `You have access to ${fields.courseName} course`,
   schema: z.object({
     orgName: z.string().min(1),
     courseName: z.string().min(1),