refactor(observability): drop file-SD writer for http_sd_configs

Slice 4 of observability-service-registry. Removes the shared-file
Prometheus bridge; external Prometheus now consumes node-exporter targets
via http_sd_configs against GET /api/monitoring/prometheus-targets.

- services/targets.py: removed write_prometheus_targets() (the file
  writer) and its json/Path/get_settings imports; updated module docstring.
  build_node_exporter_targets() is unchanged and still powers the HTTP
  endpoint.
- main.py: removed the startup write_prometheus_targets call.
- routers/settings.py: removed the _write_prometheus_targets helper and
  its three post machine create/update/delete call sites + the now-unused
  targets import.
- config.py: removed the prometheus_file_sd_dir field.
- docker-compose.yml / docker-compose.dev.yml: removed the
  PROMETHEUS_FILE_SD_DIR backend env var.
- tests: removed TestWritePrometheusTargets + the write_prometheus_targets
  import in test_targets.py; rewrote the two TestSettingsMachines tests to
  assert machines appear/disappear from /api/monitoring/prometheus-targets
  (the surviving HTTP path) instead of the removed file-writer side effect.

ruff clean; 239 backend tests pass.

backend/src/media_library_viewer_api/config.py

@@ -54,7 +54,6 @@ class Settings(BaseSettings):
 
     # Observability
     prometheus_enabled: bool = True
-    prometheus_file_sd_dir: str = "/app/backend/.cache/prometheus-file-sd"
     alertmanager_url: str = ""
     alertmanager_webhook_url: str = ""  # Optional receiver for alertmanager webhook notifications