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/main.py

@@ -13,7 +13,7 @@ from fastapi.responses import Response as FastAPIResponse
 
 from media_library_viewer_api.auth import require_jwt_auth, validate_auth_settings
 from media_library_viewer_api.config import get_settings
-from media_library_viewer_api.dependencies import get_mail_queue, get_monitoring_poller, get_settings_store
+from media_library_viewer_api.dependencies import get_mail_queue, get_settings_store
 from media_library_viewer_api.logging_utils import configure_logging, describe_settings, sanitize_log_extra
 from media_library_viewer_api.observability import (
     get_request_id,
@@ -46,13 +46,10 @@ async def lifespan(app: FastAPI):
     except Exception:
         logger.exception("Failed to write Prometheus file-SD targets during startup")
     mail_queue = get_mail_queue()
-    monitoring_poller = get_monitoring_poller()
     backup_poller = get_backup_poller()
     mail_queue.start()
-    monitoring_poller.start()
     backup_poller.start()
     yield
-    monitoring_poller.stop()
     backup_poller.stop()
     mail_queue.stop()
     logger.info("Backend shutdown complete")