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/api/import_admin.py

@@ -159,9 +159,7 @@ def _refetch_task_sync(session, task_id: int) -> dict:
         return {"status": "not_found"}
     if task.status != "failed":
         return {"status": "not_failed"}
-    settings = session.execute(
-        select(ImportSettings).where(ImportSettings.id == 1)
-    ).scalar_one()
+    settings = ImportSettings.load_sync(session)
     return attempt_refetch(session, task, Path(settings.import_scan_path))