feat(config): job intervals live in sysconfig, every setting says when it applies, the unread SponsorBlock default goes

An interval override was a row of its own table beside the one store every other admin knob uses; it
is now the `job_interval_<job>` sysconfig key (migration 0110 copies them over and drops the table).
The Scheduler page stays its one editor: the Configuration page and the setup wizard neither list nor
write those keys, since a write there would skip the reschedule.

Each ConfigSpec now states `applies` (required, no default). Every key is `live`, held by a scan that
finds no read at import time; a `restart` key is refused until the page can say so. A second scan
requires a reader for every key on the Configuration page — `sponsorblock_default` had none since it
was added, so it leaves the registry, the translations and the store.

backend/app/routes/config.py

@@ -61,7 +61,7 @@ def set_config(
     admin: User = Depends(admin_user),
     db: Session = Depends(get_db),
 ) -> dict:
-    s = sysconfig.spec(key)
+    s = sysconfig.config_page_spec(key)
     if s is None:
         raise not_found("config_key")
     if "value" not in body.model_fields_set:
@@ -100,7 +100,7 @@ def reset_config(
     admin: User = Depends(admin_user),
     db: Session = Depends(get_db),
 ) -> dict:
-    s = sysconfig.spec(key)
+    s = sysconfig.config_page_spec(key)
     if s is None:
         raise not_found("config_key")
     old = None if s.secret else sysconfig.get(db, key)