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/alembic/versions/0070_fold_maintenance_batch.py

@@ -0,0 +1,47 @@
+"""Fold the maintenance re-validation batch from app_state into sysconfig (B-core-11)
+
+The admin-tunable `maintenance_revalidate_batch` lived in its own `app_state` column with a
+hand-rolled override-or-default reader in `app.state`, duplicating the sysconfig SPECS pattern. It's
+now a sysconfig key (`system_config` store), so every admin knob lives in one place. This migration
+carries any existing override into the sysconfig store, then drops the dead column. Reversible.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0070_fold_maintenance_batch"
+down_revision: Union[str, None] = "0069_job_index_hygiene"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Preserve a customized override: copy it into the sysconfig store before the column goes.
+    op.execute(
+        """
+        INSERT INTO system_config (key, value, encrypted)
+        SELECT 'maintenance_revalidate_batch', maintenance_revalidate_batch::text, false
+        FROM app_state
+        WHERE id = 1 AND maintenance_revalidate_batch IS NOT NULL
+        ON CONFLICT (key) DO NOTHING
+        """
+    )
+    op.drop_column("app_state", "maintenance_revalidate_batch")
+
+
+def downgrade() -> None:
+    op.add_column(
+        "app_state",
+        sa.Column("maintenance_revalidate_batch", sa.Integer(), nullable=True),
+    )
+    # Carry the value back onto the row, then drop the sysconfig override (symmetric restore).
+    op.execute(
+        """
+        UPDATE app_state SET maintenance_revalidate_batch =
+            (SELECT value::int FROM system_config WHERE key = 'maintenance_revalidate_batch')
+        WHERE id = 1
+          AND EXISTS (SELECT 1 FROM system_config WHERE key = 'maintenance_revalidate_batch')
+        """
+    )
+    op.execute("DELETE FROM system_config WHERE key = 'maintenance_revalidate_batch'")