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-attachment-file-owner.service.spec.ts

@@ -0,0 +1,40 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import type { ICommentRepository } from '../../domain/repositories/comment.repository.interface.js';
+import type { IIssueRepository } from '../../../issue/domain/repositories/issue.repository.interface.js';
+import { FileOwnerRegistry } from '../../../file/application/services/file-owner.registry.js';
+import { CommentAttachmentFileOwnerService } from './comment-attachment-file-owner.service.js';
+import { buildIssue, mockCommentRepository, mockIssueRepository } from '../use-cases/test-helpers.js';
+
+describe('CommentAttachmentFileOwnerService', () => {
+  let commentRepository: ICommentRepository;
+  let issueRepository: IIssueRepository;
+  let registry: FileOwnerRegistry;
+
+  beforeEach(() => {
+    commentRepository = mockCommentRepository();
+    issueRepository = mockIssueRepository();
+    registry = new FileOwnerRegistry();
+    new CommentAttachmentFileOwnerService(commentRepository, issueRepository, registry).onModuleInit();
+  });
+
+  it('registers itself, so GC keeps files attached to comments', async () => {
+    vi.mocked(commentRepository.findReferencedFileIds).mockResolvedValue(['a']);
+
+    expect(await registry.findReferenced(['a', 'b'])).toEqual(new Set(['a']));
+  });
+
+  it('makes a file visible to users who see the issue of its comment', async () => {
+    vi.mocked(commentRepository.findIssueIdsByAttachedFile).mockResolvedValue([1]);
+    vi.mocked(issueRepository.findById).mockResolvedValue(buildIssue());
+
+    expect(await registry.isVisibleThroughAnyOwner('file-1', 'user-2')).toBe(true);
+    expect(issueRepository.findById).toHaveBeenCalledWith(1, 'user-2');
+  });
+
+  it('hides a file when the issue is not visible', async () => {
+    vi.mocked(commentRepository.findIssueIdsByAttachedFile).mockResolvedValue([1]);
+    vi.mocked(issueRepository.findById).mockResolvedValue(null);
+
+    expect(await registry.isVisibleThroughAnyOwner('file-1', 'user-3')).toBe(false);
+  });
+});