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>

.gitea/workflows/nightly.yml

@@ -85,6 +85,7 @@ jobs:
           GLITCHTIP_SECRET_KEY=${{ secrets.GLITCHTIP_SECRET_KEY }}
           GLITCHTIP_DOMAIN=https://glitchtip.archiv.raddatz.cloud
           SENTRY_DSN=${{ secrets.SENTRY_DSN }}
+          OTEL_EXPORTER_OTLP_ENDPOINT=http://tempo:4317
           EOF
 
       - name: Verify backend /import:ro mount is wired