feat: import hosts/VMs/LXC from Proxmox VE with optional auto-sync

Add a Proxmox VE importer that reads the /api2/json REST API with a read-only
API token and drops hosts (proxmox), VMs (vm) and LXC containers (lxc) onto the
canvas as typed nodes with run state and hardware specs (vCPU/RAM/disk).

- Backend: proxmox_service (httpx) + proxmox routes (test-connection, import,
  import-pending, config). Two-tier dedupe — merge onto an existing scanned node
  by IP, else synthetic pve-{host}-{vmid} identity. Update-in-place, never
  deletes. Host->guest rendered as a 'virtual' edge via the pending-link flow.
- Security: token is env-only (PROXMOX_TOKEN_*), never written to disk by the
  app, never returned by any endpoint; errors are credential-sanitized.
- Auto-sync: optional scheduled re-import into pending (APScheduler job).
- PendingDevice.properties carries specs through approve (+ migration).
- Frontend: ProxmoxImportModal, sidebar entry, pending inventory source filter,
  Settings auto-sync section, proxmoxApi client.
- Docs: docs/proxmox-import.md, README + FEATURES sections, .env.example keys.
- Tests: backend service/router/scheduler, frontend modal/client/pending.

ha-relevant: maybe

frontend/src/components/modals/__tests__/SettingsModal.test.tsx

@@ -8,9 +8,13 @@ vi.mock('@/api/client', () => ({
     get: vi.fn(),
     save: vi.fn(),
   },
+  proxmoxApi: {
+    getConfig: vi.fn(),
+    saveConfig: vi.fn(),
+  },
 }))
 
-import { settingsApi } from '@/api/client'
+import { settingsApi, proxmoxApi } from '@/api/client'
 import { toast } from 'sonner'
 import { useCanvasStore } from '@/stores/canvasStore'
 
@@ -19,6 +23,8 @@ describe('SettingsModal', () => {
     vi.clearAllMocks()
     vi.mocked(settingsApi.get).mockResolvedValue({ data: { interval_seconds: 60, service_check_enabled: false, service_check_interval: 300 } } as never)
     vi.mocked(settingsApi.save).mockResolvedValue({ data: { interval_seconds: 60, service_check_enabled: false, service_check_interval: 300 } } as never)
+    vi.mocked(proxmoxApi.getConfig).mockRejectedValue(new Error('not configured'))
+    vi.mocked(proxmoxApi.saveConfig).mockResolvedValue({ data: {} } as never)
     vi.mocked(toast.success).mockReset()
     vi.mocked(toast.error).mockReset()
   })