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/tests/test_targets.py

@@ -1,14 +1,11 @@
-"""Tests for Prometheus file-based service discovery target generation."""
+"""Tests for Prometheus Node Exporter target discovery."""
 
 from pathlib import Path
 
 import pytest
 
 from media_library_viewer_api.services.settings_store import SettingsStore
-from media_library_viewer_api.services.targets import (
-    build_node_exporter_targets,
-    write_prometheus_targets,
-)
+from media_library_viewer_api.services.targets import build_node_exporter_targets
 
 
 @pytest.fixture
@@ -88,22 +85,3 @@ class TestBuildNodeExporterTargets:
             }
         )
         assert build_node_exporter_targets(store) == []
-
-
-class TestWritePrometheusTargets:
-    def test_writes_valid_json(self, store: SettingsStore, tmp_path: Path):
-        store.upsert_machine(
-            {
-                "name": "remote1",
-                "mode": "ssh",
-                "host": "10.0.0.5",
-                "username": "u",
-                "node_exporter_enabled": True,
-                "node_exporter_port": 9200,
-            }
-        )
-        file_path = write_prometheus_targets(store, tmp_path)
-        assert file_path.exists()
-        assert file_path.name == "node_exporter_targets.json"
-        targets = build_node_exporter_targets(store)
-        assert len(targets) == 1