tsk-54: Tweak instant message endpoint to save contact and message identification (#56)

Closes #54

Reviewed-on: phoenix/textsender-api#56
Co-authored-by: phoenix <kundeng00@pm.me>
Co-committed-by: phoenix <kundeng00@pm.me>

migrations/schema.sql

@@ -39,5 +39,8 @@ CREATE TABLE IF NOT EXISTS message_event_responses (
     scheduled_message_event_id UUID NULL,
     response JSONB NOT NULL,
     user_id UUID NOT NULL,
-    sent timestamptz NOT NULL
+    sent timestamptz NOT NULL,
+    contact_id UUID NULL,
+    message_id UUID NULL,
+    status TEXT CHECK (status IN ('INSTANT', 'SCHEDULED'))
 );