feat: implement centralized taxonomy for programmatic SEO

- Create taxonomy system with oils.json (5 oils) and concerns.json (9 concerns)
- Migrate 10 content files to new data/content/oil-for-concern/ structure
- Add scripts: generate-urls.js, validate-taxonomy.js, migrate-content.js
- Update dataLoader.ts to use centralized taxonomy
- Generate 40 URLs (10 pairs × 4 languages)
- Create sitemap-programmatic.xml for SEO
- Update by-oil and by-concern directory pages

data/oil-for-concern/najbolje-arganovo-ulje-za-podocnjake.json

@@ -1,7 +1,7 @@
 {
   "slug": "najbolje-arganovo-ulje-za-podocnjake",
   "localizedSlugs": {"sr": "najbolje-arganovo-ulje-za-podocnjake", "en": "best-argan-oil-for-under-eye-bags", "de": "bestes-arganoel-fuer-auenringe", "fr": "meilleure-huile-dargan-pour-cernes"},
-  "oilSlug": "arganovo-ulje",
+  "oilSlug": "argan-oil",
   "concernSlug": "podocnjaci",
   "pageTitle": {
     "sr": "Najbolje arganovo ulje za podočnjake",