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/README.md

@@ -68,8 +68,9 @@ mutation it records, so an audit entry can never exist without its change (or vi
 - **`app.config.settings`** — boot/env config (DB URL, download root, worker concurrency): read once at
   startup, not user-tunable at runtime.
 - **`sysconfig.SPECS`** — the runtime admin knobs (DB-override-or-default). A new admin-tunable value
-  goes in `SPECS`; don't invent a fourth store. (`state.py`'s `maintenance_batch` predates this and is
-  slated to fold into `sysconfig` — do not add more like it.)
+  goes in `SPECS`; don't invent a fourth store. Each spec states when a change takes effect
+  (`applies`, required): `live` keys are read where they are used, never at import (test-held). The
+  scheduler's job intervals are `SPECS` keys too, edited on the Scheduler page, not the Configuration page.
 
 ## The gate