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>

prisma/schema.prisma

@@ -196,16 +196,17 @@ model NotificationMessage {
 /// Комментарий к задаче. При удалении автора комментарий остаётся (authorId -> NULL).
 /// deletedAt — мягкое удаление, editedAt — время последней правки текста.
 model Comment {
-  id        String    @id @default(uuid(7))
-  content   String
-  issueId   Int
-  issue     Issue     @relation(fields: [issueId], references: [id], onDelete: Cascade)
-  authorId  String?
-  author    User?     @relation(fields: [authorId], references: [id], onDelete: SetNull)
-  editedAt  DateTime? @db.Timestamptz(3)
-  deletedAt DateTime? @db.Timestamptz(3)
-  createdAt DateTime  @default(now()) @db.Timestamptz(3)
-  updatedAt DateTime  @updatedAt @db.Timestamptz(3)
+  id          String              @id @default(uuid(7))
+  content     String
+  issueId     Int
+  issue       Issue               @relation(fields: [issueId], references: [id], onDelete: Cascade)
+  authorId    String?
+  author      User?               @relation(fields: [authorId], references: [id], onDelete: SetNull)
+  editedAt    DateTime?           @db.Timestamptz(3)
+  deletedAt   DateTime?           @db.Timestamptz(3)
+  createdAt   DateTime            @default(now()) @db.Timestamptz(3)
+  updatedAt   DateTime            @updatedAt @db.Timestamptz(3)
+  attachments CommentAttachment[]
 
   @@index([issueId, createdAt])
   @@index([authorId])
@@ -226,7 +227,8 @@ model File {
   uploader   User?    @relation(fields: [uploaderId], references: [id], onDelete: SetNull)
   createdAt  DateTime @default(now()) @db.Timestamptz(3)
 
-  issueAttachments IssueAttachment[]
+  issueAttachments   IssueAttachment[]
+  commentAttachments CommentAttachment[]
 
   @@unique([storageKey])
   @@index([uploaderId])
@@ -247,3 +249,18 @@ model IssueAttachment {
   @@index([fileId])
   @@map("issue_attachment")
 }
+
+/// Вложение комментария — связь комментария с файлом. Управляется вместе
+/// с комментарием (fileIds в create/update); при мягком удалении комментария
+/// связи снимаются, чтобы файлы забрал GC.
+model CommentAttachment {
+  commentId String
+  comment   Comment  @relation(fields: [commentId], references: [id], onDelete: Cascade)
+  fileId    String
+  file      File     @relation(fields: [fileId], references: [id], onDelete: Cascade)
+  createdAt DateTime @default(now()) @db.Timestamptz(3)
+
+  @@id([commentId, fileId])
+  @@index([fileId])
+  @@map("comment_attachment")
+}