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/models.py

@@ -701,17 +701,6 @@ class AppState(Base):
     setup_token_hash: Mapped[str | None] = mapped_column(String(64))
 
 
-class SchedulerSetting(Base):
-    """Admin override for a scheduler job's run interval (minutes). One row per job id;
-    absent = use the env/config default. DB-backed so it's editable from the admin UI at
-    runtime without a redeploy (env stays a fallback)."""
-
-    __tablename__ = "scheduler_settings"
-
-    job_id: Mapped[str] = mapped_column(String(40), primary_key=True)
-    interval_minutes: Mapped[int] = mapped_column(Integer)
-
-
 class JobRun(Base):
     """One run of a background job — scheduled, an admin's "run now", or a user's ad-hoc work on one
     target — written when it starts and again when it ends (app/jobruns.py), each in its own