feat: merge IP-scanned and Proxmox-imported devices by MAC

Reconcile the same physical device discovered by both the nmap IP scan and
the Proxmox importer into a single inventory row, keyed on MAC. Previously
each path only deduped by IP, and the importer captured no MAC (and no IP for
stopped guests), so most guests double-listed.

Backend:
- mac_utils.normalize_mac: canonical MAC (lowercase, ':'-separated), the
  cross-source join key. Normalized on write and on compare.
- proxmox_service: capture the guest NIC MAC agent-free from the net0 config
  (qemu virtio=<MAC>, lxc hwaddr=<MAC>); works for stopped guests. Resolver
  now returns (ip, mac).
- proxmox persist: match existing Node/PendingDevice by ieee OR ip OR MAC;
  fill mac, keep the vm/lxc type, union sources.
- scanner persist: match PendingDevice by ip OR MAC; fill the IP a Proxmox
  import lacked, keep a pve row's type, union the scan source. Stamp query
  matches raw + normalized MAC (legacy-safe).
- Multi-source tags: new PendingDevice.discovery_sources JSON column so a
  merged device shows under both the IP and Proxmox filters. Idempotent
  migration backfills from discovery_source (legacy NULL-scalar rows with an
  IP become ["arp"]). _sources_after_merge preserves a scanned row's IP origin
  through the merge without tagging a pure Proxmox guest.
- Import now broadcasts a scan update on completion so an open inventory
  reloads without a manual refresh.

Frontend:
- pendingSources: sourceBuckets/orderedSources map discovery_sources to filter
  buckets; a device with ["arp","proxmox"] matches both filters and renders
  both badges. PendingDevicesModal filter + badges use them.

Tests: MAC normalization, config MAC capture, cross-source merge both
directions, legacy-row IP-tag preservation, no-false-IP-tag guard, refresh
broadcast, and the frontend bucket mapping.

ha-relevant: maybe

frontend/src/components/modals/PendingDeviceModal.tsx

@@ -21,6 +21,10 @@ export interface PendingDevice {
   suggested_type: string | null
   status: string
   discovery_source: string | null
+  // All sources that have observed this device (e.g. ["arp", "proxmox"]). A
+  // merged device shows under every matching filter. Falls back to
+  // [discovery_source] when absent (older rows).
+  discovery_sources?: string[]
   ieee_address?: string | null
   friendly_name?: string | null
   device_subtype?: string | null