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/tests/test_migration_0110.py

@@ -0,0 +1,56 @@
+"""Migration 0110's data step, DB-lane: an admin's interval override moves from `scheduler_settings`
+into sysconfig and back, a row for a job that no longer exists is not carried over, and the unread
+SponsorBlock default leaves the store.
+
+The schema half is checked by test_migrations.py; a value lost on the way is invisible there.
+"""
+
+from alembic import command
+from alembic.config import Config
+from sqlalchemy import text
+
+from app import scheduler
+from app.db import engine
+
+
+def _config(conn) -> dict[str, str]:
+    return dict(conn.execute(text("SELECT key, value FROM system_config")).tuples().all())
+
+
+def test_an_interval_override_moves_into_sysconfig_and_back(db):
+    cfg = Config("alembic.ini")
+    command.downgrade(cfg, "0109_job_runs")
+    try:
+        with engine.begin() as conn:
+            conn.execute(
+                text(
+                    "INSERT INTO scheduler_settings (job_id, interval_minutes)"
+                    " VALUES ('rss_poll', 25), ('log_gc', 90), ('retired_job', 5)"
+                )
+            )
+            conn.execute(
+                text(
+                    "INSERT INTO system_config (key, value, encrypted) VALUES"
+                    " ('sponsorblock_default', 'true', false), ('quota_daily_budget', '5000', false)"
+                )
+            )
+    finally:
+        command.upgrade(cfg, "head")
+
+    with engine.connect() as conn:
+        assert _config(conn) == {
+            "job_interval_rss_poll": "25",
+            "job_interval_log_gc": "90",
+            "quota_daily_budget": "5000",
+        }
+        assert conn.execute(text("SELECT to_regclass('scheduler_settings')")).scalar() is None
+    assert scheduler.load_intervals(db)["rss_poll"] == 25
+
+    command.downgrade(cfg, "0109_job_runs")
+    try:
+        with engine.connect() as conn:
+            back = conn.execute(text("SELECT job_id, interval_minutes FROM scheduler_settings"))
+            assert dict(back.tuples().all()) == {"rss_poll": 25, "log_gc": 90}
+            assert _config(conn) == {"quota_daily_budget": "5000"}
+    finally:
+        command.upgrade(cfg, "head")