feat: add slug, tags, seo_description columns to campaign_variants for blog posts

app/db/migrations/017_variant_blog_fields.sql

@@ -0,0 +1,12 @@
+-- 017_variant_blog_fields.sql
+
+-- slug: Directus URL slug. Generated from title via slugify() if NULL.
+ALTER TABLE campaign_variants ADD COLUMN slug TEXT;
+
+-- tags: JSON array of tag strings. e.g. '["self-hosting","peregrine"]'
+-- NULL = no tags.
+ALTER TABLE campaign_variants ADD COLUMN tags TEXT;
+
+-- seo_description: Short meta description for search engines (max ~160 chars).
+-- NULL = Directus default.
+ALTER TABLE campaign_variants ADD COLUMN seo_description TEXT;