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_sysconfig_registry.py

@@ -15,6 +15,9 @@ boot, sysconfig holds a stored override to it, and the admin page renders a pick
 The DB lane (`db` fixture) covers the store; the rest is pure logic.
 """
 
+import ast
+from pathlib import Path
+
 import pytest
 from pydantic import ValidationError
 
@@ -37,7 +40,8 @@ def test_each_group_is_one_run_of_keys():
 def test_every_group_sits_in_exactly_one_section():
     listed = [g for _, members in sysconfig.SECTIONS for g in members]
     assert len(listed) == len(set(listed)), listed
-    assert set(listed) == {s.group for s in sysconfig.SPECS}
+    assert set(listed) | set(sysconfig.OWN_PAGE_GROUPS) == {s.group for s in sysconfig.SPECS}
+    assert not set(listed) & set(sysconfig.OWN_PAGE_GROUPS)
     assert [sid for sid, _ in sysconfig.SECTIONS] == ["access", "youtube", "media", "system"]
 
 
@@ -112,3 +116,67 @@ def test_set_value_stores_the_canonical_spelling_and_refuses_anything_else(db):
     with pytest.raises(ValueError, match="scrape, api"):
         sysconfig.set_value(db, "search_source", "bing")
     db.rollback()
+
+
+# --- when a changed value takes effect --------------------------------------------------------------
+
+_APP = Path(__file__).resolve().parent.parent / "app"
+_READERS = {"get", "get_str", "get_int", "get_bool"}
+
+
+def _key_reads() -> list[tuple[str, str, bool]]:
+    """Every `sysconfig.get*(db, "<key>")` in the app outside the registry: (key, where, inside a
+    function)."""
+    found: list[tuple[str, str, bool]] = []
+
+    def walk(node: ast.AST, path: Path, in_fn: bool) -> None:
+        for child in ast.iter_child_nodes(node):
+            fn = in_fn or isinstance(child, (ast.FunctionDef, ast.AsyncFunctionDef, ast.Lambda))
+            if (
+                isinstance(child, ast.Call)
+                and isinstance(child.func, ast.Attribute)
+                and isinstance(child.func.value, ast.Name)
+                and child.func.value.id == "sysconfig"
+                and child.func.attr in _READERS
+                and len(child.args) >= 2
+                and isinstance(child.args[1], ast.Constant)
+                and isinstance(child.args[1].value, str)
+            ):
+                found.append((child.args[1].value, f"{path.name}:{child.lineno}", in_fn))
+            walk(child, path, fn)
+
+    for path in _APP.rglob("*.py"):
+        if path.name != "sysconfig.py":
+            walk(ast.parse(path.read_text(encoding="utf-8")), path, False)
+    return found
+
+
+def test_every_key_states_when_it_applies_and_none_needs_a_restart_yet():
+    with pytest.raises(TypeError):
+        sysconfig.ConfigSpec("k", "int", "quota", "quota_daily_budget")  # pyright: ignore[reportCallIssue]
+    # The Configuration page cannot tell the admin that a setting waits for a restart; a key that
+    # does needs that first (a badge on it and a banner once it changed).
+    assert {s.applies for s in sysconfig.SPECS} == {"live"}
+
+
+def test_a_live_key_is_read_where_it_is_used_never_at_import():
+    reads = _key_reads()
+    assert len(reads) > 50, len(
+        reads
+    )  # the scan sees the reads (it would pass vacuously otherwise)
+    assert [(k, at) for k, at, in_fn in reads if not in_fn] == []
+
+
+def test_every_key_on_the_configuration_page_has_a_reader():
+    read = {k for k, _, _ in _key_reads()}
+    page = {s.key for s in sysconfig.SPECS if s.group not in sysconfig.OWN_PAGE_GROUPS}
+    assert sorted(page - read) == []
+
+
+def test_the_intervals_are_keys_of_the_scheduler_page_only():
+    keys = {s.key for s in sysconfig.SPECS if s.group == "scheduler"}
+    assert keys == {sysconfig.interval_key(j) for j in sysconfig.JOB_INTERVAL_DEFAULTS}
+    assert all(len(k) <= 64 for k in keys)  # system_config.key
+    assert sysconfig.config_page_spec("job_interval_rss_poll") is None
+    assert sysconfig.config_page_spec("quota_daily_budget") is not None
+    assert "scheduler" not in sysconfig.describe(_FakeDb())["groups"]