tsk-51: Send message endpoint (#52)

Closes #51

Reviewed-on: phoenix/textsender-api#52
Co-authored-by: phoenix <kundeng00@pm.me>
Co-committed-by: phoenix <kundeng00@pm.me>

migrations/schema.sql

@@ -36,7 +36,7 @@ CREATE TABLE IF NOT EXISTS scheduled_message_events (
 
 CREATE TABLE IF NOT EXISTS message_event_responses (
     id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
-    scheduled_message_event_id UUID NOT NULL,
+    scheduled_message_event_id UUID NULL,
     response JSONB NOT NULL,
     user_id UUID NOT NULL,
     sent timestamptz NOT NULL