fix(ci): sync observability configs to host before docker compose up (#598)

DooD runner only shares /var/run/docker.sock — no workspace directory is
mapped to the host daemon. Relative bind mounts in
docker-compose.observability.yml resolved to paths that didn't exist on
the host; Docker auto-created directories in their place, causing
'not a directory' mount failures for all five config files.

Fix:
- docker-compose.observability.yml: replace hardcoded ./infra/observability/
  prefix with ${OBS_CONFIG_DIR:-./infra/observability} so the path is
  configurable while remaining backwards-compatible for local use.
- nightly.yml / release.yml: add a 'Sync observability configs to host'
  step that finds the job container's overlay2 MergedDir (the container's
  full filesystem as seen from the host mount namespace), then uses the
  existing nsenter/alpine pattern to cp the config tree into a stable host
  path (/srv/familienarchiv-{staging,production}/obs-configs).
  OBS_CONFIG_DIR is injected into the env file so Compose picks it up.

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

docker-compose.observability.yml

@@ -16,7 +16,7 @@ services:
     container_name: obs-prometheus
     restart: unless-stopped
     volumes:
-      - ./infra/observability/prometheus/prometheus.yml:/etc/prometheus/prometheus.yml:ro
+      - ${OBS_CONFIG_DIR:-./infra/observability}/prometheus/prometheus.yml:/etc/prometheus/prometheus.yml:ro
       - prometheus_data:/prometheus
     command:
       - '--config.file=/etc/prometheus/prometheus.yml'
@@ -79,7 +79,7 @@ services:
     container_name: obs-loki
     restart: unless-stopped
     volumes:
-      - ./infra/observability/loki/loki-config.yml:/etc/loki/loki-config.yml:ro
+      - ${OBS_CONFIG_DIR:-./infra/observability}/loki/loki-config.yml:/etc/loki/loki-config.yml:ro
       - loki_data:/loki
     command: -config.file=/etc/loki/loki-config.yml
     expose:
@@ -98,7 +98,7 @@ services:
     container_name: obs-promtail
     restart: unless-stopped
     volumes:
-      - ./infra/observability/promtail/promtail-config.yml:/etc/promtail/promtail-config.yml:ro
+      - ${OBS_CONFIG_DIR:-./infra/observability}/promtail/promtail-config.yml:/etc/promtail/promtail-config.yml:ro
       - /var/lib/docker/containers:/var/lib/docker/containers:ro
       # :ro restricts file-system access but NOT Docker API permissions — a compromised Promtail has full daemon access. Accepted risk on single-operator self-hosted archive.
       - /var/run/docker.sock:/var/run/docker.sock:ro
@@ -118,7 +118,7 @@ services:
     container_name: obs-tempo
     restart: unless-stopped
     volumes:
-      - ./infra/observability/tempo/tempo.yml:/etc/tempo.yml:ro
+      - ${OBS_CONFIG_DIR:-./infra/observability}/tempo/tempo.yml:/etc/tempo.yml:ro
       - tempo_data:/var/tempo
     command: -config.file=/etc/tempo.yml
     expose:
@@ -148,7 +148,7 @@ services:
       GF_USERS_ALLOW_SIGN_UP: "false"
     volumes:
       - grafana_data:/var/lib/grafana
-      - ./infra/observability/grafana/provisioning:/etc/grafana/provisioning:ro
+      - ${OBS_CONFIG_DIR:-./infra/observability}/grafana/provisioning:/etc/grafana/provisioning:ro
     healthcheck:
       test: ["CMD-SHELL", "wget -qO- http://localhost:3000/api/health | grep -q ok || exit 1"]
       interval: 30s