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/tasks/import_file.py

@@ -167,9 +167,7 @@ def enqueue_import(task_id: int, task_type: str) -> None:
 
 def _do_import(session, task, import_task_id: int) -> dict:
     """Actual work, called from inside the resilience wrapper."""
-    settings = session.execute(
-        select(ImportSettings).where(ImportSettings.id == 1)
-    ).scalar_one()
+    settings = ImportSettings.load_sync(session)
     import_root = Path(settings.import_scan_path)
     batch = session.get(ImportBatch, task.batch_id)
     deep = bool(batch and batch.scan_mode == "deep")