feat(scheduler): admin-tunable maintenance re-validation batch size

Expose the maintenance job's rolling re-validation batch (videos re-checked per
run) as an admin control on the Scheduler dashboard, stored in app_state
(migration 0018; NULL = the env/config default). The job reads the effective
value each run via a state helper, clamped to a sane range. Trilingual.

backend/alembic/versions/0018_maintenance_batch_setting.py

@@ -0,0 +1,30 @@
+"""admin override for the maintenance re-validation batch size
+
+Revision ID: 0018_maintenance_batch_setting
+Revises: 0017_notifications
+Create Date: 2026-06-18
+
+Adds app_state.maintenance_revalidate_batch: an admin-tunable override (from the Scheduler
+dashboard) for how many videos the maintenance job re-validates per run. NULL = use the
+env/config default. Purely additive.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0018_maintenance_batch_setting"
+down_revision: Union[str, None] = "0017_notifications"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "app_state",
+        sa.Column("maintenance_revalidate_batch", sa.Integer(), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("app_state", "maintenance_revalidate_batch")