fix(server): doc upsert without row lock (#5765)

packages/backend/server/schema.prisma

@@ -265,7 +265,9 @@ model Snapshot {
   seq         Int      @default(0) @db.Integer
   state       Bytes?   @db.ByteA
   createdAt   DateTime @default(now()) @map("created_at") @db.Timestamptz(6)
-  updatedAt   DateTime @updatedAt @map("updated_at") @db.Timestamptz(6)
+  // the `updated_at` field will not record the time of record changed,
+  // but the created time of last seen update that has been merged into snapshot.
+  updatedAt   DateTime @map("updated_at") @db.Timestamptz(6)
 
   @@id([id, workspaceId])
   @@map("snapshots")