Add attachments to comments

- CommentAttachment link table (comment <-> file) + migration
- fileIds in create/update comment; update replaces the whole set,
  synced in one transaction with the comment
- CommentDto.attachments; empty for deleted comments
- Soft delete drops attachments so GC can collect the files
- Max 10 files per comment, ordered by upload time (uuid v7)
- Registers comments as a file owner
- Rename CommentAuthorExpanderService -> CommentExpanderService
  (expands authors and files)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/modules/comment/application/services/comment-expander.service.ts

@@ -0,0 +1,38 @@
+import { Injectable } from '@nestjs/common';
+import { Comment } from '../../domain/entities/comment.entity.js';
+import { CommentDto } from '../dto/comment.dto.js';
+import { CommentApplicationMapper } from '../mappers/comment-application.mapper.js';
+import { GetUsersUseCase } from '../../../user/application/use-cases/get-users/get-users.use-case.js';
+import { GetUsersQuery } from '../../../user/application/use-cases/get-users/get-users.query.js';
+import { GetFilesUseCase } from '../../../file/application/use-cases/get-files/get-files.use-case.js';
+import { GetFilesQuery } from '../../../file/application/use-cases/get-files/get-files.query.js';
+
+/** Раскрывает автора и вложения комментария (или списка комментариев) */
+@Injectable()
+export class CommentExpanderService {
+  constructor(
+    private readonly getUsersUseCase: GetUsersUseCase,
+    private readonly getFilesUseCase: GetFilesUseCase,
+  ) {}
+
+  async expand(comment: Comment): Promise<CommentDto> {
+    const [dto] = await this.expandAll([comment]);
+    return dto!;
+  }
+
+  async expandAll(comments: Comment[]): Promise<CommentDto[]> {
+    if (comments.length === 0) return [];
+
+    const userIds = [...new Set(comments.map((comment) => comment.authorId).filter((id): id is string => Boolean(id)))];
+    const fileIds = comments.flatMap((comment) => comment.fileIds);
+
+    const [users, files] = await Promise.all([
+      userIds.length > 0 ? this.getUsersUseCase.execute(new GetUsersQuery(userIds)) : [],
+      fileIds.length > 0 ? this.getFilesUseCase.execute(new GetFilesQuery(fileIds)) : [],
+    ]);
+    const usersById = new Map(users.map((user) => [user.id, user]));
+    const filesById = new Map(files.map((file) => [file.id, file]));
+
+    return comments.map((comment) => CommentApplicationMapper.toDto(comment, usersById, filesById));
+  }
+}