Make embed proxy ports configurable via env vars for multi-instance deployments

All 13 nginx embed proxy ports (8881-8895) are now driven by environment
variables instead of being hardcoded. This prevents port conflicts when
running multiple Changemaker instances on the same host.

Chain: .env → docker-compose port mappings → nginx container env →
entrypoint.sh envsubst → services.conf.template listen directives →
API /services/config endpoint → frontend buildServiceUrl().

Existing deployments are unaffected (all vars default to current values).

Bunker Admin

api/prisma/migrations/20260322100000_add_registry_settings/migration.sql

@@ -0,0 +1,3 @@
+-- AlterTable
+ALTER TABLE "site_settings" ADD COLUMN     "gitea_registry_url" TEXT NOT NULL DEFAULT 'gitea.bnkops.com/admin',
+ADD COLUMN     "use_registry_for_upgrade" BOOLEAN NOT NULL DEFAULT false;