feat: add certificates, groups, support system, and moderation

Backend changes:
- Add Certificate generation service with beautiful HTML templates
- Add CourseGroup, GroupMember, GroupMessage models for group collaboration
- Add Homework and HomeworkSubmission models with AI + teacher grading
- Add SupportTicket and TicketMessage models for help desk
- Add Moderation API for admin/moderator course approval workflow
- All new modules: CertificatesModule, GroupsModule, SupportModule, ModerationModule

Frontend changes:
- Add certificate download button when course completed
- Update course page to load enrollment progress from backend
- Integrate lesson completion with backend API

Database schema now supports:
- Course groups with chat functionality
- Homework assignments with dual AI/human grading
- Support ticket system with admin responses
- Full moderation workflow (PENDING_REVIEW -> PUBLISHED/REJECTED)

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/web/src/lib/api.ts

@@ -294,6 +294,11 @@ class ApiClient {
     return this.request<{ data: any[]; meta: any }>(`/enrollment/${courseId}/reviews${params}`);
   }
 
+  // Certificates
+  async getCertificate(courseId: string) {
+    return this.request<{ certificateUrl: string; html: string }>(`/certificates/${courseId}`);
+  }
+
   // Search
   async searchCourses(query: string, filters?: { category?: string; difficulty?: string }) {
     const searchParams = new URLSearchParams({ q: query });