fix: Code for category

migrations/00002_create_categories_table.sql

@@ -2,6 +2,8 @@
 CREATE TABLE categories (
     id          UUID PRIMARY KEY      DEFAULT gen_random_uuid(),
     parent_id   UUID                  REFERENCES categories (id) ON DELETE RESTRICT,
+    -- Slug: lowercase latin letters, digits, and dashes. Unique.
+    code        VARCHAR(255) NOT NULL  UNIQUE,
     name        VARCHAR(255) NOT NULL,
     description TEXT         NOT NULL  DEFAULT '',
     created_at  TIMESTAMPTZ  NOT NULL  DEFAULT now(),

migrations/00003_create_datasets_table.sql

@@ -4,7 +4,6 @@ CREATE TYPE file_type AS ENUM ('vector_with_kato', 'vector', 'raster');
 CREATE TABLE datasets (
     id           UUID PRIMARY KEY       DEFAULT gen_random_uuid(),
     category_id  UUID         NOT NULL  REFERENCES categories (id) ON DELETE RESTRICT,
-    code         VARCHAR(255) NOT NULL,
     name         VARCHAR(255) NOT NULL,
     description  TEXT,
     unit         VARCHAR(255),