032: fix remaining sync→async propagation (17 call sites)

Core fixes:
  - service_datasource.py: fetch_datasource_metadata() → async
  - service.py: create_job(), update_job() → async (callers await)
  - _job_routes.py: await create_job/update_job

Maintenance scanners:
  - _dashboard_scanner.py: 4 functions → async (find_affected, _get_linked,
    _apply_filters, _resolve_title)
  - _chart_manager.py: 3 functions → async
  - _banner_renderer.py: rebuild_banner → async
  - _orchestrators.py: 3 orchestrators → async
  - maintenance_banner.py: await async calls

Migration:
  - dry_run_orchestrator.py: run(), _build_target_signatures() → async
  - risk_assessor.py: build_risks() → async
  - migration.py: await service.run()
  - mapping_service.py: sync_environment() → async

Dead code:
  - _helpers.py: _find_dashboard_id_by_slug marked DEPRECATED

backend/src/api/routes/migration.py

@@ -184,7 +184,7 @@ async def dry_run_migration(
         target_client = AsyncSupersetClient(target_env)
 
         try:
-            result = service.run(
+            result = await service.run(
                 selection=selection,
                 source_client=source_client,
                 target_client=target_client,