Reapply "Build post chat and attachment workflows"

This reverts commit 5bc4a551af.

backend/migrations/006_calendar_item_chat_messages.sql

@@ -0,0 +1,11 @@
+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);