refactor(config): fold maintenance_revalidate_batch from app_state into sysconfig (E3 S3, B-core-11)

The admin-tunable maintenance re-validation batch lived in its own app_state
column with a hand-rolled override-or-default reader in app.state — a fourth
config store duplicating the sysconfig SPECS pattern. Move it to a sysconfig key
(`maintenance` group, min=100 max=100_000): the reader (sync/maintenance.py) and
the Scheduler dashboard (routes/scheduler.py PATCH + GET) now go through
sysconfig.get_int / set_value / spec, and app.state loses get/set_maintenance_batch
+ the MIN/MAX constants + the settings import. Migration 0070 carries any existing
override into the system_config store before dropping the app_state column
(reversible; restores + copies back on downgrade). Reuses the existing
sysconfig.spec() for the dashboard's min/max, so the bounds have one source.

Full lane 365 pass (migration drift + reversibility exercised), ruff clean.

backend/app/models.py

@@ -547,9 +547,6 @@ class AppState(Base):
     sync_paused: Mapped[bool] = mapped_column(
         Boolean, default=False, server_default="false"
     )
-    # Admin override for the maintenance job's rolling re-validation batch (videos re-checked
-    # per run). NULL = use the config/env default (settings.maintenance_revalidate_batch).
-    maintenance_revalidate_batch: Mapped[int | None] = mapped_column(Integer)
     # First-run install wizard. `configured` is False on a fresh instance (the app runs in
     # setup mode: only the wizard works) and flipped True when the wizard finishes. While
     # unconfigured, a one-time setup token (only its SHA-256 hash stored) gates the wizard;