Add message endpoint (#13)

Reviewed-on: phoenix/textsender_api#13

migrations/20260613211440_init.sql

@@ -10,3 +10,9 @@ CREATE TABLE IF NOT EXISTS "contacts" (
     lastname TEXT NULL,
     nickname TEXT NULL
 );
+
+CREATE TABLE IF NOT EXISTS "messages" (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    content TEXT NOT NULL,
+    user_id UUID NOT NULL
+);