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

.env.example

@@ -46,3 +46,15 @@ MCP_SERVICE_KEY=svc_changeme
 # the same value in the `X-API-Key` header.
 # Generate: python3 -c "import secrets; print(secrets.token_urlsafe(32))"
 # HOMEPAGE_API_KEY=
+
+# Proxmox VE import — pull hosts/VMs/LXC from the Proxmox REST API.
+# The token is a credential: kept in memory only, never written to disk by the
+# app, never returned by any API. Create it under Datacenter → Permissions →
+# API Tokens and grant the read-only PVEAuditor role at path "/".
+# Token id format: user@realm!tokenname (e.g. root@pam!homelable).
+# Required only for auto-sync; one-off imports can pass the token in the dialog.
+# PROXMOX_TOKEN_ID=root@pam!homelable
+# PROXMOX_TOKEN_SECRET=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
+# PROXMOX_HOST=192.168.1.10
+# PROXMOX_PORT=8006
+# PROXMOX_VERIFY_TLS=true          # set false only for self-signed certs