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/database.py

@@ -115,6 +115,8 @@ async def init_db() -> None:
             await conn.exec_driver_sql("ALTER TABLE nodes ADD COLUMN right_handles INTEGER NOT NULL DEFAULT 0")
         with suppress(OperationalError):
             await conn.exec_driver_sql("ALTER TABLE pending_devices ADD COLUMN discovery_source TEXT")
+        with suppress(OperationalError):
+            await conn.exec_driver_sql("ALTER TABLE pending_devices ADD COLUMN properties JSON")
         with suppress(OperationalError):
             await conn.exec_driver_sql("ALTER TABLE scan_runs ADD COLUMN kind TEXT NOT NULL DEFAULT 'ip'")
         # --- Zigbee schema migrations (logged variant per CLAUDE.md feedback) ---
@@ -162,6 +164,7 @@ async def init_db() -> None:
                     "model VARCHAR,"
                     "vendor VARCHAR,"
                     "lqi INTEGER,"
+                    "properties JSON,"
                     "discovered_at DATETIME"
                     ")"
                 )