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

backend/app/main.py

@@ -7,7 +7,21 @@ from typing import Any
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
-from app.api.routes import auth, canvas, designs, edges, liveview, media, nodes, scan, stats, status, zigbee, zwave
+from app.api.routes import (
+    auth,
+    canvas,
+    designs,
+    edges,
+    liveview,
+    media,
+    nodes,
+    proxmox,
+    scan,
+    stats,
+    status,
+    zigbee,
+    zwave,
+)
 from app.api.routes import settings as settings_routes
 from app.core.config import settings
 from app.core.scheduler import start_scheduler, stop_scheduler
@@ -58,6 +72,7 @@ app.include_router(settings_routes.router, prefix="/api/v1/settings", tags=["set
 app.include_router(liveview.router, prefix="/api/v1/liveview", tags=["liveview"])
 app.include_router(zigbee.router, prefix="/api/v1/zigbee", tags=["zigbee"])
 app.include_router(zwave.router, prefix="/api/v1/zwave", tags=["zwave"])
+app.include_router(proxmox.router, prefix="/api/v1/proxmox", tags=["proxmox"])
 app.include_router(stats.router, prefix="/api/v1/stats", tags=["stats"])
 app.include_router(media.router, prefix="/api/v1/media", tags=["media"])