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/db/models.py

@@ -119,7 +119,13 @@ class PendingDevice(Base):
     services: Mapped[list[Any]] = mapped_column(JSON, default=list)
     suggested_type: Mapped[str | None] = mapped_column(String)
     status: Mapped[str] = mapped_column(String, default="pending")
+    # Origin/primary source (first discovery): "arp"/"mdns"/"zigbee"/"zwave"/
+    # "proxmox". Kept for back-compat; `discovery_sources` is the full set.
     discovery_source: Mapped[str | None] = mapped_column(String)
+    # All sources that have observed this device. A device found by both an IP
+    # scan and a Proxmox import carries e.g. ["arp", "proxmox"] and shows under
+    # both inventory filters. Source of truth for the frontend source badges.
+    discovery_sources: Mapped[list[Any]] = mapped_column(JSON, default=list)
     ieee_address: Mapped[str | None] = mapped_column(String, index=True, nullable=True, unique=True)
     friendly_name: Mapped[str | None] = mapped_column(String, nullable=True)
     device_subtype: Mapped[str | None] = mapped_column(String, nullable=True)