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>

packages/database/prisma/schema.prisma

@@ -42,6 +42,11 @@ model User {
   purchases       Purchase[]
   reviews         Review[]
   generations     CourseGeneration[]
+  groupMembers    GroupMember[]
+  groupMessages   GroupMessage[]
+  homeworkSubmissions HomeworkSubmission[]
+  supportTickets  SupportTicket[]
+  ticketMessages  TicketMessage[]
   
   @@map("users")
 }
@@ -183,6 +188,7 @@ model Course {
   purchases       Purchase[]
   reviews         Review[]
   generation      CourseGeneration?
+  groups          CourseGroup[]
   
   // Vector embedding for semantic search
   embedding       Unsupported("vector(1536)")?
@@ -234,6 +240,7 @@ model Lesson {
   
   // Relations
   chapter     Chapter  @relation(fields: [chapterId], references: [id], onDelete: Cascade)
+  homework    Homework[]
   
   // Vector embedding for semantic search
   embedding   Unsupported("vector(1536)")?
@@ -431,3 +438,137 @@ model LessonProgress {
   @@index([enrollmentId])
   @@map("lesson_progress")
 }
+
+// ============================================
+// Course Groups & Collaboration
+// ============================================
+
+model CourseGroup {
+  id          String   @id @default(uuid())
+  courseId    String   @map("course_id")
+  name        String
+  description String?  @db.Text
+  
+  createdAt   DateTime @default(now()) @map("created_at")
+  updatedAt   DateTime @updatedAt @map("updated_at")
+  
+  course      Course   @relation(fields: [courseId], references: [id], onDelete: Cascade)
+  members     GroupMember[]
+  messages    GroupMessage[]
+  
+  @@index([courseId])
+  @@map("course_groups")
+}
+
+model GroupMember {
+  id        String   @id @default(uuid())
+  groupId   String   @map("group_id")
+  userId    String   @map("user_id")
+  role      String   @default("student") // "teacher", "student"
+  
+  joinedAt  DateTime @default(now()) @map("joined_at")
+  
+  group     CourseGroup @relation(fields: [groupId], references: [id], onDelete: Cascade)
+  user      User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+  
+  @@unique([groupId, userId])
+  @@map("group_members")
+}
+
+model GroupMessage {
+  id        String   @id @default(uuid())
+  groupId   String   @map("group_id")
+  userId    String   @map("user_id")
+  content   String   @db.Text
+  
+  createdAt DateTime @default(now()) @map("created_at")
+  
+  group     CourseGroup @relation(fields: [groupId], references: [id], onDelete: Cascade)
+  user      User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+  
+  @@index([groupId])
+  @@map("group_messages")
+}
+
+// ============================================
+// Homework & Assignments
+// ============================================
+
+model Homework {
+  id              String   @id @default(uuid())
+  lessonId        String   @map("lesson_id")
+  title           String
+  description     String   @db.Text
+  dueDate         DateTime? @map("due_date")
+  
+  createdAt       DateTime @default(now()) @map("created_at")
+  updatedAt       DateTime @updatedAt @map("updated_at")
+  
+  lesson          Lesson   @relation(fields: [lessonId], references: [id], onDelete: Cascade)
+  submissions     HomeworkSubmission[]
+  
+  @@index([lessonId])
+  @@map("homework")
+}
+
+model HomeworkSubmission {
+  id            String   @id @default(uuid())
+  homeworkId    String   @map("homework_id")
+  userId        String   @map("user_id")
+  content       String   @db.Text
+  
+  // AI grading
+  aiScore       Int?     @map("ai_score") // 0-100
+  aiFeedback    String?  @db.Text @map("ai_feedback")
+  
+  // Teacher grading
+  teacherScore  Int?     @map("teacher_score") // 0-100
+  teacherFeedback String? @db.Text @map("teacher_feedback")
+  
+  submittedAt   DateTime @default(now()) @map("submitted_at")
+  gradedAt      DateTime? @map("graded_at")
+  
+  homework      Homework @relation(fields: [homeworkId], references: [id], onDelete: Cascade)
+  user          User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+  
+  @@unique([homeworkId, userId])
+  @@map("homework_submissions")
+}
+
+// ============================================
+// Support Tickets
+// ============================================
+
+model SupportTicket {
+  id          String   @id @default(uuid())
+  userId      String   @map("user_id")
+  title       String
+  status      String   @default("open") // "open", "in_progress", "resolved", "closed"
+  priority    String   @default("normal") // "low", "normal", "high"
+  
+  createdAt   DateTime @default(now()) @map("created_at")
+  updatedAt   DateTime @updatedAt @map("updated_at")
+  
+  user        User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+  messages    TicketMessage[]
+  
+  @@index([userId])
+  @@index([status])
+  @@map("support_tickets")
+}
+
+model TicketMessage {
+  id        String   @id @default(uuid())
+  ticketId  String   @map("ticket_id")
+  userId    String   @map("user_id")
+  content   String   @db.Text
+  isStaff   Boolean  @default(false) @map("is_staff")
+  
+  createdAt DateTime @default(now()) @map("created_at")
+  
+  ticket    SupportTicket @relation(fields: [ticketId], references: [id], onDelete: Cascade)
+  user      User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+  
+  @@index([ticketId])
+  @@map("ticket_messages")
+}