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/routers/settings.py

@@ -17,7 +17,6 @@ from media_library_viewer_api.services.db_maintenance import remove_sqlite_datab
 from media_library_viewer_api.services.known_hosts import has_known_host
 from media_library_viewer_api.services.media_index import MediaIndex
 from media_library_viewer_api.services.settings_store import SettingsStore
-from media_library_viewer_api.services.targets import write_prometheus_targets
 
 logger = logging.getLogger(__name__)
 
@@ -121,14 +120,6 @@ def _validate_saved_machine_ssh(machine: MonitoringMachineInput, store: Settings
         client.close()
 
 
-def _write_prometheus_targets(store: SettingsStore) -> None:
-    """Regenerate Prometheus file-SD targets after machine changes."""
-    try:
-        write_prometheus_targets(store)
-    except Exception:
-        logger.exception("Failed to write Prometheus file-SD targets")
-
-
 @router.post("/machines/test-ssh")
 def test_machine_ssh(
     machine: MonitoringMachineInput,
@@ -189,7 +180,6 @@ def post_machine(
     store: SettingsStore = Depends(get_settings_store),
 ) -> dict[str, Any]:
     saved = store.upsert_machine(machine.model_dump(exclude_none=True), machine.id)
-    _write_prometheus_targets(store)
     saved_machine = MonitoringMachineInput.model_validate(saved)
     _validate_saved_machine_ssh(saved_machine, store)
     return saved
@@ -204,7 +194,6 @@ def put_machine(
     if not store.get_machine(machine_id):
         raise HTTPException(status_code=404, detail="Machine not found")
     saved = store.upsert_machine(machine.model_dump(exclude_none=True), machine_id)
-    _write_prometheus_targets(store)
     saved_machine = MonitoringMachineInput.model_validate(saved)
     _validate_saved_machine_ssh(saved_machine, store)
     return saved
@@ -215,7 +204,6 @@ def delete_machine(machine_id: str, store: SettingsStore = Depends(get_settings_
     if not store.get_machine(machine_id):
         raise HTTPException(status_code=404, detail="Machine not found")
     store.delete_machine(machine_id)
-    _write_prometheus_targets(store)
     return {"status": "deleted"}