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/state.py

@@ -4,16 +4,11 @@ import secrets
 
 from sqlalchemy.orm import Session
 
-from app.config import settings
 from app.models import AppState
 from app.security import hash_token
 
 log = logging.getLogger("siftlode.state")
 
-# Bounds for the admin-set maintenance re-validation batch (videos re-checked per run).
-MAINTENANCE_BATCH_MIN = 100
-MAINTENANCE_BATCH_MAX = 100_000
-
 # Once configuration completes in this process it never reverts, so cache it to skip the per-request
 # DB hit in the setup-mode gate (see main.setup_gate). Starts False; refreshed from the DB.
 _configured_cache = False
@@ -90,20 +85,3 @@ def check_setup_token(db: Session, raw: str | None) -> bool:
         return False
     stored = _row(db).setup_token_hash
     return stored is not None and secrets.compare_digest(stored, hash_token(raw))
-
-
-def get_maintenance_batch(db: Session) -> int:
-    """Effective maintenance re-validation batch: the admin override if set, else the
-    env/config default."""
-    return _row(db).maintenance_revalidate_batch or settings.maintenance_revalidate_batch
-
-
-def set_maintenance_batch(db: Session, value: int) -> int:
-    """Clamp and persist the maintenance batch override. Returns the applied value."""
-    value = max(MAINTENANCE_BATCH_MIN, min(MAINTENANCE_BATCH_MAX, int(value)))
-    row = _row(db)
-    row.maintenance_revalidate_batch = value
-    db.add(row)
-    db.commit()
-    log.info("Maintenance re-validation batch set to %s", value)
-    return value