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/backup.py

@@ -246,9 +246,7 @@ def prune_backups() -> dict:
     SessionLocal = _sync_session_factory()
     counts = {"db_deleted": 0, "images_deleted": 0, "files_unlinked": 0}
     with SessionLocal() as session:
-        s = session.execute(
-            select(ImportSettings).where(ImportSettings.id == 1)
-        ).scalar_one()
+        s = ImportSettings.load_sync(session)
         for kind, keep in (
             ("db", s.backup_db_keep_last_n),
             ("images", s.backup_images_keep_last_n),
@@ -286,9 +284,7 @@ def backup_db_nightly() -> dict:
     either {'skipped': '<reason>'} or {'dispatched': '<task_id>'}."""
     SessionLocal = _sync_session_factory()
     with SessionLocal() as session:
-        s = session.execute(
-            select(ImportSettings).where(ImportSettings.id == 1)
-        ).scalar_one()
+        s = ImportSettings.load_sync(session)
         nightly_enabled = s.backup_db_nightly_enabled
         configured_hour = s.backup_db_nightly_hour_utc
     if not nightly_enabled: