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/dependencies.py

@@ -21,12 +21,6 @@ from media_library_viewer_api.clients.ssh import RemoteSSHClient
 from media_library_viewer_api.config import get_settings
 from media_library_viewer_api.services.mail_queue import MailQueue
 from media_library_viewer_api.services.mail_queue import get_mail_queue as _get_mail_queue
-from media_library_viewer_api.services.monitoring_poller import (
-    MonitoringPoller,
-)
-from media_library_viewer_api.services.monitoring_poller import (
-    get_monitoring_poller as _get_monitoring_poller,
-)
 from media_library_viewer_api.services.settings_store import SettingsStore
 from media_library_viewer_api.services.settings_store import get_settings_store as _get_settings_store
 
@@ -251,11 +245,6 @@ def get_mail_queue() -> MailQueue:
     return _get_mail_queue()
 
 
-def get_monitoring_poller() -> MonitoringPoller:
-    """Return the singleton background monitoring poller."""
-    return _get_monitoring_poller()
-
-
 def get_settings_store() -> SettingsStore:
     """Return the singleton persistent settings store."""
     return _get_settings_store()