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/use-cases/update-comment/update-comment.use-case.spec.ts

@@ -10,10 +10,13 @@ import { UpdateCommentCommand } from './update-comment.command.js';
 import {
   buildComment,
   buildIssue,
-  mockAuthorExpander,
+  mockCommentExpander,
   mockCommentRepository,
+  mockFileAccess,
   mockIssueRepository,
 } from '../test-helpers.js';
+import { FileNotFoundException } from '../../../../file/domain/exceptions/file-not-found.exception.js';
+import type { FileAccessService } from '../../../../file/application/services/file-access.service.js';
 
 describe('UpdateCommentUseCase', () => {
   let commentRepository: ICommentRepository;
@@ -21,16 +24,19 @@ describe('UpdateCommentUseCase', () => {
   let commentEventsService: CommentEventsService;
   let useCase: UpdateCommentUseCase;
   let received: unknown[];
+  let fileAccess: FileAccessService;
 
   beforeEach(() => {
     commentRepository = mockCommentRepository();
     issueRepository = mockIssueRepository();
     commentEventsService = new CommentEventsService();
+    fileAccess = mockFileAccess({ gone: new FileNotFoundException('gone') });
     useCase = new UpdateCommentUseCase(
       commentRepository,
       new CommentAccessService(commentRepository, issueRepository),
-      mockAuthorExpander(),
+      mockCommentExpander(),
       commentEventsService,
+      fileAccess,
     );
     received = [];
     commentEventsService.onEvents().subscribe((event) => received.push(event));
@@ -82,4 +88,25 @@ describe('UpdateCommentUseCase', () => {
       CommentNotFoundException,
     );
   });
+
+  it('replaces attachments, checking only newly added files', async () => {
+    const comment = buildComment();
+    comment.edit('Original', 'user-1', ['kept', 'removed']);
+    vi.mocked(commentRepository.findById).mockResolvedValue(comment);
+
+    await useCase.execute(new UpdateCommentCommand('comment-1', 'user-1', 'Original', ['kept', 'new']));
+
+    expect(fileAccess.getAttachableFile).toHaveBeenCalledExactlyOnceWith('new', 'user-1');
+    expect(vi.mocked(commentRepository.update).mock.calls[0]![0].fileIds).toEqual(['kept', 'new']);
+    expect(received).toHaveLength(1);
+  });
+
+  it('does not save when an added file is gone', async () => {
+    vi.mocked(commentRepository.findById).mockResolvedValue(buildComment());
+
+    await expect(
+      useCase.execute(new UpdateCommentCommand('comment-1', 'user-1', 'Original', ['gone'])),
+    ).rejects.toThrow(FileNotFoundException);
+    expect(commentRepository.update).not.toHaveBeenCalled();
+  });
 });