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/monitoring.py

@@ -1,18 +1,14 @@
-"""Monitoring router — disk checks, action history, and observability stack status."""
+"""Monitoring router — observability stack status (Alertmanager + Prometheus)."""
 
 from __future__ import annotations
 
 import logging
 from typing import Any
 
-from fastapi import APIRouter, Body, Depends, HTTPException, Query
+from fastapi import APIRouter, Body, Depends
 
 from media_library_viewer_api.config import get_settings
 from media_library_viewer_api.dependencies import get_settings_store
-from media_library_viewer_api.services.monitoring_actions import (
-    disk_space,
-    run_machine_operation,
-)
 from media_library_viewer_api.services.settings_store import SettingsStore
 from media_library_viewer_api.services.targets import build_node_exporter_targets
 
@@ -68,80 +64,12 @@ def _summary_from_alerts(alerts: list[dict[str, Any]]) -> dict[str, Any]:
 router = APIRouter(prefix="/api/monitoring", tags=["monitoring"])
 
 
-def _resolve_machine(store: SettingsStore, machine_id: str | None) -> dict[str, Any]:
-    """Return the requested machine or the first enabled machine.
-
-    Monitoring is treated as a machine-by-machine view. If a machine is
-    explicitly requested but disabled, we surface that as a user-facing error so
-    the Settings tab can be used to re-enable it.
-    """
-    machines = store.list_machines()
-    if machine_id:
-        machine = next((item for item in machines if item["id"] == machine_id), None)
-        if not machine:
-            raise HTTPException(status_code=404, detail="Monitoring machine not found")
-        if not machine.get("enabled"):
-            raise HTTPException(status_code=409, detail=f"Monitoring machine '{machine['name']}' is disabled")
-        return machine
-
-    for machine in machines:
-        if machine.get("enabled"):
-            return machine
-    raise HTTPException(status_code=404, detail="No enabled monitoring machines configured")
-
-
 @router.get("/machines")
 def get_machines(store: SettingsStore = Depends(get_settings_store)) -> list[dict[str, Any]]:
     """Return enabled monitoring machines for the UI."""
     return [m for m in store.list_machines() if m.get("enabled")]
 
 
-@router.get("/poller")
-def get_poller_status() -> dict[str, Any]:
-    """Return the backend poller status and configuration."""
-    from media_library_viewer_api.dependencies import get_monitoring_poller
-
-    poller = get_monitoring_poller().snapshot()
-    logger.info(
-        "Monitoring poller status requested running=%s poll_count=%s",
-        poller.get("worker_running"),
-        poller.get("poll_count"),
-    )
-    return poller
-
-
-@router.get("/machines/{machine_id}/actions")
-def get_machine_actions(
-    machine_id: str,
-    limit: int = 20,
-    action: str | None = None,
-    status: str | None = None,
-    store: SettingsStore = Depends(get_settings_store),
-) -> dict[str, Any]:
-    """Return recent action history for a single machine."""
-    machine = _resolve_machine(store, machine_id)
-    actions = store.list_machine_actions(machine["id"], limit=limit, action=action, status=status)
-    return {"items": actions, "total": len(actions)}
-
-
-@router.get("/disk")
-def get_disk_space(
-    machine_id: str | None = Query(default=None),
-    store: SettingsStore = Depends(get_settings_store),
-) -> dict[str, Any]:
-    """Return disk space for the configured path of a given machine."""
-    machine = _resolve_machine(store, machine_id)
-    app_settings = get_settings()
-    path = str(machine.get("media_root") or app_settings.media_root or "/")
-    logger.info("Monitoring disk requested machine_id=%s path=%s", machine["id"], path)
-    return run_machine_operation(
-        machine,
-        store,
-        f"disk lookup for {path}",
-        lambda client: disk_space(client, path),
-    )
-
-
 @router.get("/prometheus-targets")
 def get_prometheus_targets(store: SettingsStore = Depends(get_settings_store)) -> list[dict[str, Any]]:
     """Return Prometheus file-SD targets for remote Node Exporters.