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>

package.json

@@ -6,7 +6,8 @@
     "dev": "next dev",
     "build": "next build",
     "start": "next start",
-    "lint": "eslint"
+    "lint": "eslint",
+    "db:upload-assets": "node --env-file=.env.local scripts/upload-assets.mjs"
   },
   "dependencies": {
     "@supabase/supabase-js": "^2.117.1",