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-ulje-slatkog-badema-za-osetljivu-kozu.json

@@ -1,7 +1,7 @@
 {
   "slug": "najbolje-ulje-slatkog-badema-za-osetljivu-kozu",
   "localizedSlugs": {"sr": "najbolje-ulje-slatkog-badema-za-osetljivu-kozu", "en": "best-sweet-almond-oil-for-sensitive-skin", "de": "bestes-suessmandeloel-fuer-empfindliche-haut", "fr": "meilleure-huile-damande-douce-pour-peau-sensible"},
-  "oilSlug": "ulje-slatkog-badema",
+  "oilSlug": "sweet-almond-oil",
   "concernSlug": "osetljiva-koza",
   "pageTitle": {
     "sr": "Najbolje ulje slatkog badema za osetljivu kožu",