fix(scheduler): address /code-review R3 findings on the maintenance handler (3)

Round-3 review of the S3 config-fold, all in routes/scheduler.py:
- stale docstring: "Persisted to app_state" -> "Persisted via sysconfig" (the
  batch moved stores in B-core-11; the comment didn't follow).
- drop the manual min/max check in the PATCH — sysconfig.set_value already
  validates against the same spec bounds; catch its ValueError -> 400, matching
  the config PATCH pattern (bounds enforced in one place).
- fetch sysconfig.spec(...) once into a local in the GET instead of twice.

No behaviour change beyond the 400 message wording (untested path). 368 pass.

backend/app/routes/scheduler.py

@@ -90,14 +90,12 @@ def set_maintenance_settings(
     db: Session = Depends(get_db),
 ) -> 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."""
-    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 {s.min} and {s.max}",
-        )
-    sysconfig.set_value(db, "maintenance_revalidate_batch", body.revalidate_batch)
+    run). Persisted via sysconfig; the job reads it each run."""
+    # set_value validates against the spec's min/max (one source of bounds), mirroring the config PATCH.
+    try:
+        sysconfig.set_value(db, "maintenance_revalidate_batch", body.revalidate_batch)
+    except (TypeError, ValueError) as exc:
+        raise HTTPException(status_code=400, detail=str(exc) or "Invalid value")
     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()
@@ -165,6 +163,7 @@ def get_scheduler(
         "daily_budget": sysconfig.get_int(db, "quota_daily_budget"),
         "backfill_reserve": sysconfig.get_int(db, "backfill_quota_reserve"),
     }
+    maint = sysconfig.spec("maintenance_revalidate_batch")
 
     return {
         **snap,
@@ -174,7 +173,7 @@ def get_scheduler(
         "maintenance": {
             "revalidate_batch": sysconfig.get_int(db, "maintenance_revalidate_batch"),
             "revalidate_batch_default": settings.maintenance_revalidate_batch,
-            "min": sysconfig.spec("maintenance_revalidate_batch").min,
-            "max": sysconfig.spec("maintenance_revalidate_batch").max,
+            "min": maint.min,
+            "max": maint.max,
         },
     }