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/presentation/dto/update-comment.dto.ts

@@ -1,9 +1,18 @@
-import { IsNotEmpty, IsString, MaxLength } from 'class-validator';
+import { ArrayMaxSize, IsArray, IsNotEmpty, IsOptional, IsString, MaxLength } from 'class-validator';
 import { CommentContent } from '../../domain/value-objects/comment-content.vo.js';
+import { Comment } from '../../domain/entities/comment.entity.js';
 
 export class UpdateCommentRequestDto {
   @IsString()
   @IsNotEmpty()
   @MaxLength(CommentContent.MAX_LENGTH)
   content: string;
+
+  /** id файлов, загруженных через POST /files — набор целиком; не передан — не менять */
+  @IsOptional()
+  @IsArray()
+  @ArrayMaxSize(Comment.MAX_ATTACHMENTS)
+  @IsString({ each: true })
+  @MaxLength(64, { each: true })
+  fileIds?: string[];
 }