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/database.py

@@ -97,6 +97,8 @@ async def init_db() -> None:
             await conn.exec_driver_sql("ALTER TABLE nodes ADD COLUMN bottom_handles INTEGER NOT NULL DEFAULT 1")
         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 scan_runs ADD COLUMN kind TEXT NOT NULL DEFAULT 'ip'")
         # --- Zigbee schema migrations (logged variant per CLAUDE.md feedback) ---
         zigbee_migrations: list[tuple[str, str]] = [
             ("nodes.ieee_address", "ALTER TABLE nodes ADD COLUMN ieee_address TEXT"),