Lets a chat message reply to another, through chat_messages.reply_to_id.
The column has no foreign key on purpose: when the replied-to message is
deleted, the id stays and chat-service's join reads null, so the chat shows
"deleted message" where the quote was.
f
up(db: DatabaseSync): void
Run once by runMigrations. Existing messages reply to nothing, and a
database made from schema.sql already has the column.
Usage
import * as mod from "api/db/migrations/0009_chat_reply.ts";