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/download.py

@@ -3,7 +3,6 @@
 import asyncio
 from pathlib import Path
 
-from sqlalchemy import select
 from sqlalchemy.exc import DBAPIError, OperationalError
 
 from ..celery_app import celery
@@ -41,9 +40,7 @@ def download_source(self, source_id: int) -> int:
         SyncFactory = _sync_session_factory()
         try:
             with SyncFactory() as sync_session:
-                settings = sync_session.execute(
-                    select(ImportSettings).where(ImportSettings.id == 1)
-                ).scalar_one()
+                settings = ImportSettings.load_sync(sync_session)
                 rate_limit = settings.download_rate_limit_seconds
                 validate_files = settings.download_validate_files
 
@@ -57,9 +54,7 @@ def download_source(self, source_id: int) -> int:
             async with async_factory() as async_session:
                 cred_service = CredentialService(async_session, crypto)
                 with SyncFactory() as sync_session:
-                    sync_settings = sync_session.execute(
-                        select(ImportSettings).where(ImportSettings.id == 1)
-                    ).scalar_one()
+                    sync_settings = ImportSettings.load_sync(sync_session)
                     importer = Importer(
                         session=sync_session,
                         images_root=IMAGES_ROOT,