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/scheduler_service.py

@@ -58,9 +58,7 @@ async def select_due_sources(session: AsyncSession) -> list[Source]:
         .where(Artist.auto_check.is_(True))
     )).scalars().all()
 
-    settings = (await session.execute(
-        select(ImportSettings).where(ImportSettings.id == 1)
-    )).scalar_one()
+    settings = await ImportSettings.load(session)
 
     now = datetime.now(UTC)
     due: list[Source] = []
@@ -121,9 +119,7 @@ async def scheduler_status(session: AsyncSession) -> dict:
         .where(Source.enabled.is_(True))
         .where(Artist.auto_check.is_(True))
     )).scalars().all()
-    settings = (await session.execute(
-        select(ImportSettings).where(ImportSettings.id == 1)
-    )).scalar_one()
+    settings = await ImportSettings.load(session)
 
     now = datetime.now(UTC)
     due_now = 0