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/sysconfig.py

@@ -59,6 +59,10 @@ SPECS: tuple[ConfigSpec, ...] = (
     # --- Batch sizes ---
     ConfigSpec("enrich_batch_size", "int", "batch", "enrich_batch_size", min=1, max=50),
     ConfigSpec("autotag_title_sample", "int", "batch", "autotag_title_sample", min=1, max=1_000),
+    # --- Maintenance (rolling re-validation of already-known videos) ---
+    # Videos re-checked per maintenance run. Folded here from app_state (B-core-11) so every admin
+    # knob lives in one store; read by sync/maintenance.py, tuned from the Scheduler dashboard.
+    ConfigSpec("maintenance_revalidate_batch", "int", "maintenance", "maintenance_revalidate_batch", min=100, max=100_000),
     # --- Discovery cleanup (search + explore ephemeral content) ---
     # Days an explored-but-unsubscribed channel is kept before the cleanup job purges it.
     ConfigSpec("explore_grace_days", "int", "explore", "explore_grace_days", min=0, max=3_650),