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.

frontend/src/i18n/locales/en/config.json

@@ -162,10 +162,6 @@
       "label": "Worker concurrency",
       "hint": "How many downloads the worker processes in parallel."
     },
-    "sponsorblock_default": {
-      "label": "SponsorBlock by default",
-      "hint": "Enable SponsorBlock (skip sponsor segments) for newly created custom download profiles."
-    },
     "audit_retention_days": {
       "label": "Retention (days)",
       "hint": "How long admin audit-log entries are kept before the audit-log cleanup job prunes them. 0 = keep forever. Default 180."

frontend/src/i18n/locales/hu/config.json

@@ -162,10 +162,6 @@
       "label": "Párhuzamos letöltések",
       "hint": "Hány letöltést dolgoz fel egyszerre a letöltő folyamat."
     },
-    "sponsorblock_default": {
-      "label": "SponsorBlock alapból",
-      "hint": "SponsorBlock (szponzorszakaszok átugrása) bekapcsolása az újonnan létrehozott egyéni letöltési profilokhoz."
-    },
     "audit_retention_days": {
       "label": "Megőrzés (nap)",
       "hint": "Meddig maradnak meg az admin-naplóbejegyzések, mielőtt az eseménynapló-takarító feladat törli őket. 0 = örökre. Alapérték 180."