fix(staging): correct backend healthcheck port and OTel endpoint

Two bugs introduced when the management port was split from the app port:

1. Backend healthcheck hit localhost:8080/actuator/health (app port) —
   actuator is on management.server.port=8081, so every probe got a 404
   from the main MVC dispatcher, marking the container permanently unhealthy.
   Fix: change the probe to localhost:8081.

2. OTEL_EXPORTER_OTLP_ENDPOINT was not set in .env.staging, so the exporter
   fell back to http://localhost:4317 (the CI-safe default) instead of
   http://tempo:4317 (the in-network Tempo service). Fix: inject the correct
   endpoint in the nightly env-file generation step.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.prod.yml

@@ -216,7 +216,7 @@ services:
     networks:
       - archiv-net
     healthcheck:
-      test: ["CMD-SHELL", "wget -qO- http://localhost:8080/actuator/health | grep -q UP || exit 1"]
+      test: ["CMD-SHELL", "wget -qO- http://localhost:8081/actuator/health | grep -q UP || exit 1"]
       interval: 15s
       timeout: 5s
       retries: 10