feat!(migrations): init migration has unique constraint on column slug

db/migrations/001_trainings.up.sql

@@ -5,7 +5,7 @@ CREATE SCHEMA IF NOT EXISTS training;
 CREATE TABLE IF NOT EXISTS training.trainings (
   id          SERIAL PRIMARY KEY,
   name        varchar(255) NOT NULL,
-  slug        varchar(255) NOT NULL,
+  slug        varchar(255) UNIQUE NOT NULL,
   description text NOT NULL,
   days        smallint NOT NULL
 );