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/PendingDeviceModal.tsx

@@ -1,6 +1,7 @@
 import { Globe, Router, Server, Layers, Box, Container, HardDrive, Cpu, Wifi, Circle, Network } from 'lucide-react'
 import { Dialog, DialogContent, DialogHeader, DialogTitle } from '@/components/ui/dialog'
 import { Button } from '@/components/ui/button'
+import type { NodeProperty } from '@/types'
 
 interface Service {
   port: number
@@ -26,6 +27,8 @@ export interface PendingDevice {
   model?: string | null
   vendor?: string | null
   lqi?: number | null
+  // Display properties carried from discovery (e.g. Proxmox specs).
+  properties?: NodeProperty[]
   discovered_at: string
   // How many canvases (designs) this device already appears on. Computed server-side.
   canvas_count?: number