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

backend/app/services/mac_utils.py

@@ -0,0 +1,18 @@
+"""MAC-address normalization, shared by the scan + Proxmox persist paths.
+
+Different discovery sources emit MACs in different casing/separators (ARP is
+lowercase ``bc:24:11:..``, Proxmox config is often uppercase ``BC:24:11:..``).
+Canonicalizing on write *and* on compare lets cross-source dedup match a device
+by MAC with a plain ``==`` — the join key for merging an IP-scanned row with a
+Proxmox-imported one.
+"""
+
+from __future__ import annotations
+
+
+def normalize_mac(mac: str | None) -> str | None:
+    """Canonical MAC: lowercase, ``-`` → ``:``, stripped. Blank/None → None."""
+    if not mac:
+        return None
+    normalized = mac.strip().lower().replace("-", ":")
+    return normalized or None