refactor(dry-B2): ImportSettings.load()/load_sync() classmethods for the singleton row

The `select(ImportSettings).where(id == 1)).scalar_one()` singleton load was
repeated 15× across services, API, and 5 task modules. Added async load() +
sync load_sync() classmethods on the model and migrated all 15 full-row sites
(callers already imported ImportSettings, so no new imports; dropped download's
now-orphaned select import). Left maintenance.py's deliberate column-select
(import_scan_path only) as-is.

Rest of the service layer was already adequately DRY — the Record/to_dict
pattern is only 2 instances and the savepoint find-or-create recovery is
correctly per-entity, so neither was forced into a shared abstraction.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/services/source_service.py

@@ -120,9 +120,7 @@ class SourceService:
         return config
 
     async def _load_settings(self) -> ImportSettings:
-        return (await self.session.execute(
-            select(ImportSettings).where(ImportSettings.id == 1)
-        )).scalar_one()
+        return await ImportSettings.load(self.session)
 
     def _build_record(
         self, source: Source, artist: Artist, settings: ImportSettings,