fix(deploy): wire SENTRY_DSN and enable ECS JSON logging for prod (#641)

Pass SENTRY_DSN env var through to the backend container so the Sentry SDK
actually ships exceptions to GlitchTip — the variable was written to
.env.staging by nightly.yml but never forwarded into the container.

Enable Spring Boot 4.0 ECS structured logging (LOGGING_STRUCTURED_FORMAT_CONSOLE=ecs)
so Loki receives single-entry JSON log lines with parsed log.level, enabling
detected_level filtering in Grafana instead of 50-line unlinked stack trace blobs.

Update Grafana Loki dashboard query from | logfmt to | json to match the new format.

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

docker-compose.prod.yml

@@ -252,6 +252,8 @@ services:
       OTEL_METRICS_EXPORTER: none
       MANAGEMENT_METRICS_TAGS_APPLICATION: Familienarchiv
       MANAGEMENT_TRACING_SAMPLING_PROBABILITY: ${MANAGEMENT_TRACING_SAMPLING_PROBABILITY:-0.1}
+      SENTRY_DSN: ${SENTRY_DSN:-}
+      LOGGING_STRUCTURED_FORMAT_CONSOLE: ecs
     networks:
       - archiv-net
     healthcheck: