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.
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