feat(zigbee): import as background scan run

Mirrors IP scan flow: POST /zigbee/import-pending now creates a
ScanRun(kind=zigbee, status=running) and returns immediately.
Networkmap fetch + pending upsert run in the background, status
transitions to done/error when finished.

Frontend: import modal closes on submit, scan history shows the
run with a ZIG/IP kind chip and toasts on completion. Pending
modal auto-refreshes when run finishes.

scan_runs.kind column added (default 'ip', idempotent migration).
Existing zigbee tests refactored to exercise _persist_pending_import
directly (background tasks don't see the test session); route test
verifies the run is created with kind=zigbee.

backend/app/db/models.py

@@ -128,6 +128,7 @@ class ScanRun(Base):
 
     id: Mapped[str] = mapped_column(String, primary_key=True, default=_uuid)
     status: Mapped[str] = mapped_column(String, default="running")
+    kind: Mapped[str] = mapped_column(String, default="ip", server_default="ip")
     ranges: Mapped[list[str]] = mapped_column(JSON, default=list)
     devices_found: Mapped[int] = mapped_column(Integer, default=0)
     started_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=_now)