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/services/targets.py

@@ -1,19 +1,16 @@
-"""Prometheus file-based service discovery target management.
+"""Prometheus Node Exporter target discovery.
 
 The backend owns the list of remote Node Exporter targets so that operators can
-enable scraping per machine from the Manage UI.  Prometheus reads the generated
-JSON file via `file_sd_configs`; this keeps Prometheus config static and pushes
-machine-specific changes into a file it can reload.
+enable scraping per machine from the Manage UI. The list is exposed over HTTP at
+``GET /api/monitoring/prometheus-targets`` and consumed by an external Prometheus
+via ``http_sd_configs`` (no shared volume required).
 """
 
 from __future__ import annotations
 
-import json
 import logging
-from pathlib import Path
 from typing import Any
 
-from media_library_viewer_api.config import get_settings
 from media_library_viewer_api.services.settings_store import SettingsStore
 
 logger = logging.getLogger(__name__)
@@ -34,10 +31,9 @@ def _scrape_address(machine: dict[str, Any]) -> str | None:
 
 
 def build_node_exporter_targets(store: SettingsStore) -> list[dict[str, Any]]:
-    """Build a file-SD target list for all enabled SSH machines.
+    """Build an http-SD target list for all enabled SSH machines.
 
-    Local machines are excluded because the Compose-managed node-exporter
-    service already covers the Docker host.
+    Local machines are excluded because the Docker host is scraped directly.
     """
     targets: list[dict[str, Any]] = []
     for machine in store.list_machines():
@@ -60,18 +56,3 @@ def build_node_exporter_targets(store: SettingsStore) -> list[dict[str, Any]]:
             }
         )
     return targets
-
-
-def write_prometheus_targets(store: SettingsStore, file_sd_dir: Path | None = None) -> Path:
-    """Render and persist Prometheus file-SD targets.
-
-    Returns the path written so callers can log or expose it.
-    """
-    settings = get_settings()
-    file_sd_dir = file_sd_dir or Path(settings.prometheus_file_sd_dir)
-    file_sd_dir.mkdir(parents=True, exist_ok=True)
-    file_path = file_sd_dir / "node_exporter_targets.json"
-    targets = build_node_exporter_targets(store)
-    file_path.write_text(json.dumps(targets, indent=2), encoding="utf-8")
-    logger.info("Wrote %s node_exporter targets to %s", len(targets), file_path)
-    return file_path