Revert "Build post chat and attachment workflows"

This reverts commit 0b920da187.

backend/migrations/006_calendar_item_chat_messages.sql

@@ -1,11 +0,0 @@
-CREATE TABLE IF NOT EXISTS calendar_item_chat_messages (
-  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
-  calendar_item_id UUID NOT NULL REFERENCES calendar_items(id) ON DELETE CASCADE,
-  channel TEXT NOT NULL CHECK (channel IN ('external', 'internal')),
-  body TEXT NOT NULL,
-  created_by UUID REFERENCES users(id) ON DELETE SET NULL,
-  created_at TIMESTAMPTZ NOT NULL DEFAULT now()
-);
-
-CREATE INDEX IF NOT EXISTS idx_calendar_item_chat_messages_item_channel
-  ON calendar_item_chat_messages (calendar_item_id, channel, created_at);