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/list-comments/list-comments.use-case.spec.ts

@@ -7,7 +7,7 @@ import { ListCommentsQuery } from './list-comments.query.js';
 import {
   buildComment,
   buildIssue,
-  mockAuthorExpander,
+  mockCommentExpander,
   mockCommentRepository,
   mockIssueRepository,
 } from '../test-helpers.js';
@@ -20,7 +20,7 @@ describe('ListCommentsUseCase', () => {
   beforeEach(() => {
     commentRepository = mockCommentRepository();
     issueRepository = mockIssueRepository();
-    useCase = new ListCommentsUseCase(commentRepository, issueRepository, mockAuthorExpander());
+    useCase = new ListCommentsUseCase(commentRepository, issueRepository, mockCommentExpander());
   });
 
   it('forwards pagination params and returns the next cursor', async () => {