Add Comment model for issue comments

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

prisma/schema.prisma

@@ -24,6 +24,7 @@ model User {
   notificationDeviceTokens NotificationDeviceToken[]
   notificationPreferences  NotificationPreference[]
   notificationMessages     NotificationMessage[]
+  comments                 Comment[]
 
   @@unique([email])
   @@map("user")
@@ -92,6 +93,7 @@ model Issue {
   projectId  String
   project    Project   @relation(fields: [projectId], references: [id], onDelete: Cascade)
   dueDate    DateTime? @db.Timestamptz(3)
+  comments   Comment[]
   createdAt  DateTime  @default(now()) @db.Timestamptz(3)
   updatedAt  DateTime  @updatedAt @db.Timestamptz(3)
 
@@ -188,3 +190,22 @@ model NotificationMessage {
   @@index([createdAt])
   @@map("notification_message")
 }
+
+/// 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)
+
+  @@index([issueId, createdAt])
+  @@index([authorId])
+  @@map("comment")
+}