[Backend] Create scheduled course launch and waitlist API

[SYLVIANNORUKA]

Description: Implement API for courses with a future launch date, allowing students to join a waitlist beforehand.

Requirements:

• Handle waitlist signup requests
• Track waitlist position per course
• Validate course launch date before enabling enrollment
• Manage automatic notification at launch
• Provide waitlist status to student

Acceptance Criteria:

• Signup requests are handled
• Waitlist position is tracked
• Launch date is validated before enrollment
• Automatic notification works
• Waitlist status is provided to student

Technical Details:

Files: backend/src/modules/waitlist/waitlist.controller.ts, backend/src/modules/waitlist/waitlist.service.ts, prisma/schema.prisma
Libraries: @nestjs/common, @nestjs/schedule, prisma