feat(admin): apix-admin UI at admin.api-index.org

Internal maintainer dashboard: global stats, sandbox list with
tier-filter and pagination, sandbox drill-down with D3 world map,
tier-promotion form. API-key auth via HttpOnly cookie. Port 8084,
rolling a/b deploy alongside the existing service pairs.

- apix-common: AdminSandboxSummary, AdminSandboxListResponse, AdminStatsResponse DTOs
- apix-registry: AdminResource (4 endpoints), SandboxService.listAllSandboxes + getGlobalStats
- apix-registry: SandboxResource GET /{uuid}/dashboard endpoint (was missing — fixes portal 500)
- apix-portal: RegistryClient path corrected to /dashboard
- apix-admin: new Quarkus module — auth filter, registry REST client, 3 resources, 6 Qute templates, admin.js
- infra: Dockerfile.admin, docker-compose admin-a/b services, Caddyfile admin.api-index.org block
- scripts: deploy-bluegreen.sh extended for admin-a/b

Co-Authored-By: Mira Rehfeld <noreply@anthropic.com>

infra/Dockerfile.admin

@@ -0,0 +1,8 @@
+FROM eclipse-temurin:21-jre-alpine
+RUN addgroup -S apix && adduser -S apix -G apix
+WORKDIR /app
+COPY apix-admin/target/quarkus-app/ quarkus-app/
+RUN chown -R apix:apix /app
+USER apix
+EXPOSE 8084
+ENTRYPOINT ["java", "-jar", "quarkus-app/quarkus-run.jar"]