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/routes/scheduler.py

@@ -91,16 +91,14 @@ def set_maintenance_settings(
 ) -> dict:
     """Admin-tune the maintenance job's rolling re-validation batch (videos re-checked per
     run). Persisted to app_state; the job reads it each run."""
-    value = body.revalidate_batch
-    if not (state.MAINTENANCE_BATCH_MIN <= value <= state.MAINTENANCE_BATCH_MAX):
+    s = sysconfig.spec("maintenance_revalidate_batch")
+    if not (s.min <= body.revalidate_batch <= s.max):
         raise HTTPException(
             status_code=400,
-            detail=(
-                f"revalidate_batch must be between {state.MAINTENANCE_BATCH_MIN} and "
-                f"{state.MAINTENANCE_BATCH_MAX}"
-            ),
+            detail=f"revalidate_batch must be between {s.min} and {s.max}",
         )
-    new = state.set_maintenance_batch(db, value)
+    sysconfig.set_value(db, "maintenance_revalidate_batch", body.revalidate_batch)
+    new = sysconfig.get_int(db, "maintenance_revalidate_batch")
     audit.record(db, admin.id, AuditAction.MAINTENANCE_TUNE, summary=f"revalidate batch = {new}", after={"revalidate_batch": new})
     db.commit()
     return {"revalidate_batch": new}
@@ -174,9 +172,9 @@ def get_scheduler(
         "queue": queue,
         "quota": quota_info,
         "maintenance": {
-            "revalidate_batch": state.get_maintenance_batch(db),
+            "revalidate_batch": sysconfig.get_int(db, "maintenance_revalidate_batch"),
             "revalidate_batch_default": settings.maintenance_revalidate_batch,
-            "min": state.MAINTENANCE_BATCH_MIN,
-            "max": state.MAINTENANCE_BATCH_MAX,
+            "min": sysconfig.spec("maintenance_revalidate_batch").min,
+            "max": sysconfig.spec("maintenance_revalidate_batch").max,
         },
     }