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/system_backup.py

@@ -227,9 +227,7 @@ async def delete_run(run_id: int):
 @system_backup_bp.route("/settings", methods=["GET"])
 async def get_settings():
     async with get_session() as session:
-        row = (await session.execute(
-            select(ImportSettings).where(ImportSettings.id == 1)
-        )).scalar_one()
+        row = await ImportSettings.load(session)
     return jsonify({
         "backup_db_nightly_enabled": row.backup_db_nightly_enabled,
         "backup_db_nightly_hour_utc": row.backup_db_nightly_hour_utc,
@@ -249,9 +247,7 @@ async def patch_settings():
         return err
 
     async with get_session() as session:
-        row = (await session.execute(
-            select(ImportSettings).where(ImportSettings.id == 1)
-        )).scalar_one()
+        row = await ImportSettings.load(session)
         for field in _BACKUP_SETTINGS_FIELDS:
             if field in body:
                 setattr(row, field, body[field])