fix(docker): add SUPABASE_INTERNAL_URL for server-side Supabase access

Server-side code (proxy.ts, SSR, API routes) now uses SUPABASE_INTERNAL_URL
(http://supabase-kong:8000) instead of the external domain. This avoids
hairpin NAT / DNS resolution issues where Docker containers can't reach
their own external domain through the reverse proxy.

Browser-side JS still uses the external URL (baked at build time).

docker-compose.yml

@@ -321,10 +321,12 @@ services:
     environment:
       NODE_ENV: production
       NEXT_PUBLIC_SITE_URL: ${SITE_URL:-http://localhost:3000}
-      # Must match the build-time value — server code reads from process.env
+      # Browser-side: external domain (baked at build time, re-stated here for SSR)
       NEXT_PUBLIC_SUPABASE_URL: ${API_EXTERNAL_URL:-http://localhost:8000}
       NEXT_PUBLIC_SUPABASE_PUBLIC_KEY: ${SUPABASE_ANON_KEY}
       NEXT_PUBLIC_DEFAULT_LOCALE: de
+      # Server-side: Docker-internal URL (avoids hairpin NAT / DNS issues)
+      SUPABASE_INTERNAL_URL: http://supabase-kong:8000
       SUPABASE_SECRET_KEY: ${SUPABASE_SERVICE_ROLE_KEY}
       SUPABASE_DB_WEBHOOK_SECRET: ${DB_WEBHOOK_SECRET:-webhooksecret}
       EMAIL_SENDER: ${EMAIL_SENDER:-noreply@myeasycms.de}