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/db/models.py

@@ -126,6 +126,10 @@ class PendingDevice(Base):
     model: Mapped[str | None] = mapped_column(String, nullable=True)
     vendor: Mapped[str | None] = mapped_column(String, nullable=True)
     lqi: Mapped[int | None] = mapped_column(Integer, nullable=True)
+    # Display properties carried from discovery (e.g. Proxmox specs: CPU/RAM/Disk,
+    # VMID). Generic NodeProperty shape {key,value,icon,visible}; merged into the
+    # Node's properties on approve. Empty for scan/mesh sources that don't set it.
+    properties: Mapped[list[Any]] = mapped_column(JSON, default=list)
     discovered_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=_now)
 
     # Transient (not persisted): populated per-request by the scan routes to report