refactor(monitoring): decommission legacy SSH-scraping poller (slice 1)

The 2026-06-16/17 observability update externalised metrics to
Prometheus + node_exporter + Grafana, but the legacy Manage-side
SSH-scraping monitor was never removed. It duplicated the new stack,
ran SSH df on every machine every 300s, and fed nothing (its UI was
deleted in e2ad731). This slice decommissions the duplication.

Removed (backend):
- services/monitoring_poller.py (MonitoringPoller) — entire file
- services/monitoring_actions.py (disk_space, run_machine_operation,
  poll_machine_snapshot, build_machine_client) — entire file;
  run_machine_operation had only 2 callers (the poller + /disk), both gone
- tests/test_monitoring_actions.py
- endpoints: POST /api/monitoring/poller, GET /machines/{id}/actions,
  GET /disk (and the now-dead _resolve_machine helper)
- lifespan wiring (main.py), dependency wrapper (dependencies.py),
  poller.start()/kick() from machine save (routers/settings.py)
- SettingsStore: monitoring_machine_actions table CREATE + 2 indexes +
  record/list/prune_machine_actions methods; DROP TABLE IF EXISTS on
  startup cleans existing DBs (user-approved)
- config knobs: monitoring_poll_interval_seconds,
  monitoring_poll_initial_delay_seconds, monitoring_action_retention_days
- test_api.py: TestMonitoring._ensure_machine + test_disk

Kept (fits the new model): /machines, /prometheus-targets, /alerts,
/alertmanager-status, /alertmanager-webhook; the disk_usage JOB template
(manual on-demand, not monitoring); node_exporter_* machine fields
(they point Prometheus at the right host).

Gate: backend pytest 173 passed; ruff clean.

backend/src/media_library_viewer_api/routers/settings.py

@@ -12,7 +12,7 @@ from pydantic import BaseModel, Field
 
 from media_library_viewer_api.clients.ssh import RemoteSSHClient
 from media_library_viewer_api.config import get_settings
-from media_library_viewer_api.dependencies import get_monitoring_poller, get_settings_store
+from media_library_viewer_api.dependencies import get_settings_store
 from media_library_viewer_api.services.db_maintenance import remove_sqlite_database
 from media_library_viewer_api.services.known_hosts import has_known_host
 from media_library_viewer_api.services.media_index import MediaIndex
@@ -195,13 +195,8 @@ def post_machine(
 ) -> dict[str, Any]:
     saved = store.upsert_machine(machine.model_dump(exclude_none=True), machine.id)
     _write_prometheus_targets(store)
-    poller = get_monitoring_poller()
-    try:
-        saved_machine = MonitoringMachineInput.model_validate(saved)
-        _validate_saved_machine_ssh(saved_machine, store)
-    finally:
-        poller.start()
-        poller.kick()
+    saved_machine = MonitoringMachineInput.model_validate(saved)
+    _validate_saved_machine_ssh(saved_machine, store)
     return saved
 
 
@@ -215,13 +210,8 @@ def put_machine(
         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)
-    poller = get_monitoring_poller()
-    try:
-        saved_machine = MonitoringMachineInput.model_validate(saved)
-        _validate_saved_machine_ssh(saved_machine, store)
-    finally:
-        poller.start()
-        poller.kick()
+    saved_machine = MonitoringMachineInput.model_validate(saved)
+    _validate_saved_machine_ssh(saved_machine, store)
     return saved