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/settings.py

@@ -31,9 +31,7 @@ _EDITABLE_FIELDS = (
 @settings_bp.route("/settings/import", methods=["GET"])
 async def get_import_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({
         "min_width": row.min_width,
         "min_height": row.min_height,
@@ -99,9 +97,7 @@ async def update_import_settings():
             return _bad_int("download_failure_warning_threshold", 1, 100)
 
     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 _EDITABLE_FIELDS:
             if field in body:
                 setattr(row, field, body[field])