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/domain/entities/comment.entity.spec.ts

@@ -73,4 +73,50 @@ describe('Comment', () => {
       expect(() => buildComment().delete('user-2')).toThrow(CommentForbiddenException);
     });
   });
+
+  describe('attachments', () => {
+    it('creates a comment with deduplicated files', () => {
+      const comment = Comment.create({ issueId: 1, authorId: 'user-1', content: 'Hi', fileIds: ['a', 'b', 'a'] });
+
+      expect(comment.fileIds).toEqual(['a', 'b']);
+    });
+
+    it('limits the number of files', () => {
+      const fileIds = Array.from({ length: Comment.MAX_ATTACHMENTS + 1 }, (_, i) => `f${i}`);
+
+      expect(() => Comment.create({ issueId: 1, authorId: 'user-1', content: 'Hi', fileIds })).toThrow();
+    });
+
+    it('treats a changed file set as an edit, even with the same text', () => {
+      const comment = buildComment();
+
+      expect(comment.edit('Original', 'user-1', ['a'])).toBe(true);
+      expect(comment.fileIds).toEqual(['a']);
+      expect(comment.editedAt).toBeInstanceOf(Date);
+    });
+
+    it('keeps files when fileIds is not passed', () => {
+      const comment = buildComment();
+      comment.edit('Original', 'user-1', ['a']);
+
+      expect(comment.edit('Changed', 'user-1')).toBe(true);
+      expect(comment.fileIds).toEqual(['a']);
+    });
+
+    it('reports no change for the same text and files', () => {
+      const comment = buildComment();
+      comment.edit('Original', 'user-1', ['a', 'b']);
+
+      expect(comment.edit('Original', 'user-1', ['a', 'b'])).toBe(false);
+    });
+
+    it('drops files on delete so GC can collect them', () => {
+      const comment = buildComment();
+      comment.edit('Original', 'user-1', ['a']);
+
+      comment.delete('user-1');
+
+      expect(comment.fileIds).toEqual([]);
+    });
+  });
 });