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/core/mapping_service.py

@@ -73,7 +73,8 @@ class IdMappingService:
                 for env_id in environments:
                     client = superset_client_factory(env_id)
                     if client:
-                        self.sync_environment(env_id, client)
+                        import asyncio
+                        asyncio.run(self.sync_environment(env_id, client))
             self._sync_job = self.scheduler.add_job(
                 sync_all,
                 CronTrigger.from_crontab(cron_string),
@@ -96,7 +97,7 @@ class IdMappingService:
     # @PARAM    superset_client - Instance capable of hitting the Superset API.
     # @PRE  environment_id exists in the database.
     # @POST  ResourceMapping records for the environment are created or updated.
-    def sync_environment(
+    async def sync_environment(
         self, environment_id: str, superset_client, incremental: bool = False
     ) -> None:
         """
@@ -150,7 +151,7 @@ class IdMappingService:
                                 logger.debug(
                                     f"[IdMappingService.sync_environment] Incremental sync since {since_dttm}"
                                 )
-                        resources = superset_client.get_all_resources(
+                        resources = await superset_client.get_all_resources(
                             endpoint, since_dttm=since_dttm
                         )
                         # Track which UUIDs we see in this sync cycle