tsk-9: Schedule queued message endpoint (#22)

Closes #9

Reviewed-on: phoenix/textsender-api#22
Co-authored-by: phoenix <kundeng00@pm.me>
Co-committed-by: phoenix <kundeng00@pm.me>

migrations/schema.sql

@@ -13,3 +13,11 @@ CREATE TABLE messages (
     content TEXT NOT NULL,
     user_id UUID NOT NULL
 );
+
+CREATE TABLE scheduled_messages (
+    id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+    scheduled timestamptz NOT NULL,
+    created timestamptz DEFAULT now(),
+    status TEXT CHECK (status IN ('PENDING', 'READY', 'PROCESSING', 'DONE')),
+    user_id UUID NOT NULL
+);