feat: add ticket comments, file attachments, mention notifications

- Comment module with CRUD and file upload via multer
- @mention parsing with user notification
- Assignment notification on ticket update
- Notification service enhanced with ticketId linking
- ServeStaticModule for uploaded files

src/comment/dto/create-comment.dto.ts

@@ -0,0 +1,7 @@
+import { IsString, MinLength } from 'class-validator';
+
+export class CreateCommentDto {
+  @IsString()
+  @MinLength(1)
+  content!: string;
+}