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/api/src/groups/groups.service.ts

@@ -0,0 +1,48 @@
+import { Injectable, NotFoundException, ForbiddenException } from '@nestjs/common';
+import { PrismaService } from '../common/prisma/prisma.service';
+
+@Injectable()
+export class GroupsService {
+  constructor(private prisma: PrismaService) {}
+
+  async createGroup(courseId: string, userId: string, name: string, description?: string): Promise<any> {
+    const course = await this.prisma.course.findFirst({ where: { id: courseId, authorId: userId } });
+    if (!course) throw new ForbiddenException('Only course author can create groups');
+
+    return this.prisma.courseGroup.create({
+      data: { courseId, name, description },
+    });
+  }
+
+  async addMember(groupId: string, userId: string, role = 'student'): Promise<any> {
+    return this.prisma.groupMember.create({
+      data: { groupId, userId, role },
+    });
+  }
+
+  async getGroupMessages(groupId: string, userId: string): Promise<any> {
+    const member = await this.prisma.groupMember.findUnique({
+      where: { groupId_userId: { groupId, userId } },
+    });
+    if (!member) throw new ForbiddenException('Not a member of this group');
+
+    return this.prisma.groupMessage.findMany({
+      where: { groupId },
+      include: { user: { select: { id: true, name: true, avatarUrl: true } } },
+      orderBy: { createdAt: 'asc' },
+      take: 100,
+    });
+  }
+
+  async sendMessage(groupId: string, userId: string, content: string): Promise<any> {
+    const member = await this.prisma.groupMember.findUnique({
+      where: { groupId_userId: { groupId, userId } },
+    });
+    if (!member) throw new ForbiddenException('Not a member of this group');
+
+    return this.prisma.groupMessage.create({
+      data: { groupId, userId, content },
+      include: { user: { select: { id: true, name: true, avatarUrl: true } } },
+    });
+  }
+}