chore: zalozenie databazy a uloziska na novom serveri

- supabase/schema.sql: idempotentny skript (tabulky, RLS, bucket newsletter-assets)
- supabase/assets + npm run db:upload-assets: staticke obrazky sablony
- adresa uloziska sa sklada z NEXT_PUBLIC_SUPABASE_URL namiesto pevneho projektu
- .env.example a navod na instalaciu v README

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/lib/defaultCampaign.ts

@@ -1,7 +1,8 @@
 import { NewsletterCampaign, ProductItem } from '@/types/newsletter';
 
-export const SUPABASE_ASSETS_BASE =
-  'https://dqexxfspkylqttrtxsww.supabase.co/storage/v1/object/public/newsletter-assets/';
+// Verejné úložisko statických obrázkov (logá, ikony, štítky predajní...) v Supabase projekte z .env.
+// Súbory sú v supabase/assets/ a nahrajú sa príkazom: npm run db:upload-assets
+export const SUPABASE_ASSETS_BASE = `${(process.env.NEXT_PUBLIC_SUPABASE_URL || '').replace(/\/$/, '')}/storage/v1/object/public/newsletter-assets/`;
 
 export const DEFAULT_CAMPAIGN: NewsletterCampaign = {
   id: '',