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/migration/dry_run_orchestrator.py

@@ -54,7 +54,7 @@ class MigrationDryRunService:
     # @PRE  source/target clients are authenticated and selection validated by caller.
     # @POST  Returns JSON-serializable pre-flight payload with summary, diff and risk.
     # @SIDE_EFFECT  Reads source export archives and target metadata via network.
-    def run(
+    async def run(
         self,
         selection: DashboardSelection,
         source_client: SupersetClient,
@@ -73,14 +73,14 @@ class MigrationDryRunService:
                 else {}
             )
             transformed = {"dashboards": {}, "charts": {}, "datasets": {}}
-            dashboards_preview = source_client.get_dashboards_summary()
+            dashboards_preview = await source_client.get_dashboards_summary()
             selected_preview = {
                 item["id"]: item
                 for item in dashboards_preview
                 if item.get("id") in selection.selected_ids
             }
             for dashboard_id in selection.selected_ids:
-                exported_content, _ = source_client.export_dashboard(int(dashboard_id))
+                exported_content, _ = await source_client.export_dashboard(int(dashboard_id))
                 with create_temp_file(
                     content=exported_content, suffix=".zip"
                 ) as source_zip, create_temp_file(suffix=".zip") as transformed_zip:
@@ -103,7 +103,7 @@ class MigrationDryRunService:
             source_objects = {
                 key: list(value.values()) for key, value in transformed.items()
             }
-            target_objects = self._build_target_signatures(target_client)
+            target_objects = await self._build_target_signatures(target_client)
             diff = {
                 "dashboards": self._build_object_diff(
                     source_objects["dashboards"], target_objects["dashboards"]
@@ -115,7 +115,7 @@ class MigrationDryRunService:
                     source_objects["datasets"], target_objects["datasets"]
                 ),
             }
-            risk = self._build_risks(
+            risk = await self._build_risks(
                 source_objects, target_objects, diff, target_client
             )
             summary = {
@@ -219,10 +219,10 @@ class MigrationDryRunService:
     # #endregion _build_object_diff
     # #region _build_target_signatures [TYPE Function]
     # @PURPOSE: Pull target metadata and normalize it into comparable signatures.
-    def _build_target_signatures(
+    async def _build_target_signatures(
         self, client: SupersetClient
     ) -> dict[str, list[dict[str, Any]]]:
-        _, dashboards = client.get_dashboards(
+        _, dashboards = await client.get_dashboards(
             query={
                 "columns": [
                     "uuid",
@@ -235,7 +235,7 @@ class MigrationDryRunService:
                 ],
             }
         )
-        _, datasets = client.get_datasets(
+        _, datasets = await client.get_datasets(
             query={
                 "columns": [
                     "uuid",
@@ -248,7 +248,7 @@ class MigrationDryRunService:
                 ],
             }
         )
-        _, charts = client.get_charts(
+        _, charts = await client.get_charts(
             query={
                 "columns": [
                     "uuid",
@@ -330,14 +330,14 @@ class MigrationDryRunService:
     # #endregion _build_target_signatures
     # #region _build_risks [TYPE Function]
     # @PURPOSE: Build risk items for missing datasource, broken refs, overwrite, owner mismatch.
-    def _build_risks(
+    async def _build_risks(
         self,
         source_objects: dict[str, list[dict[str, Any]]],
         target_objects: dict[str, list[dict[str, Any]]],
         diff: dict[str, dict[str, list[dict[str, Any]]]],
         target_client: SupersetClient,
     ) -> list[dict[str, Any]]:
-        return build_risks(source_objects, target_objects, diff, target_client)
+        return await build_risks(source_objects, target_objects, diff, target_client)
     # #endregion _build_risks
 # #endregion MigrationDryRunService
 # #endregion MigrationDryRunOrchestratorModule

backend/src/core/migration/risk_assessor.py

@@ -96,7 +96,7 @@ def extract_owner_identifiers(owners: Any) -> list[str]:
 # @DATA_CONTRACT    Dict[str, Dict[str, List[Dict[str, Any]]]],
 # @DATA_CONTRACT    SupersetClient
 # @DATA_CONTRACT  ) -> List[Dict[str, Any]]
-def build_risks(
+async def build_risks(
     source_objects: dict[str, list[dict[str, Any]]],
     target_objects: dict[str, list[dict[str, Any]]],
     diff: dict[str, dict[str, list[dict[str, Any]]]],
@@ -118,7 +118,7 @@ def build_risks(
                 )
 
         target_dataset_uuids = set(index_by_uuid(target_objects["datasets"]).keys())
-        _, target_databases = target_client.get_databases(query={"columns": ["uuid"]})
+        _, target_databases = await target_client.get_databases(query={"columns": ["uuid"]})
         target_database_uuids = {
             str(item.get("uuid")) for item in target_databases if item.get("uuid")
         }