autoresearch iter 18: remove the phantom user:daily_audit schedule

The user plugin registered a cron dispatching to user:daily_audit, a task
pattern it never registers, and no audit logic exists anywhere in the plugin.
The daily run therefore went nowhere while a test asserted the schedule was
registered — proving the wiring existed, not that it worked. Implementing a
real daily audit is unstarted functionality, so the schedule is removed rather
than stubbed.

The combined domain test now asserts the real invariant across all applied
plugins: every schedule's task type must have a registered handler.

backend/plugins/domain/domain_test.go

@@ -234,10 +234,6 @@ func TestUserPlugin(t *testing.T) {
 	require.True(t, ok)
 	assert.Equal(t, 3, taskDef.Retry)
 
-	schedDef, ok := ctx.Schedules().Get("user:daily_audit")
-	require.True(t, ok)
-	assert.Equal(t, "0 3 * * *", schedDef.Spec)
-
 	// 10. Settings
 	sReg, ok := ctx.Settings().Get("user.registration_enabled")
 	require.True(t, ok)
@@ -433,7 +429,15 @@ func TestAllDomainPluginsCombined(t *testing.T) {
 
 	// Verify total schedules registered
 	allSchedules := ctx.Schedules().Schedules()
-	assert.GreaterOrEqual(t, len(allSchedules), 3)
+	assert.GreaterOrEqual(t, len(allSchedules), 2)
+
+	// 每个调度指向的任务类型都必须已注册 Handler，否则触发时会投递到无人处理的
+	// 任务类型，预期的清理逻辑静默失效。
+	for _, sched := range allSchedules {
+		_, ok := ctx.Tasks().Get(sched.TaskType)
+		assert.Truef(t, ok, "schedule %q dispatches to task %q, which is never registered",
+			sched.Spec, sched.TaskType)
+	}
 
 	// Verify total settings schemas registered
 	allSettings := ctx.Settings().Schemas()

backend/plugins/domain/user/plugin.go

@@ -138,10 +138,7 @@ func (p *Plugin) Apply(ctx *core.Context) error {
 		return nil
 	})
 
-	// 5. Register Cron Schedules
-	ctx.Schedule().RegisterCron("0 3 * * *", "user:daily_audit", map[string]string{"type": "audit"})
-
-	// 6. Register Settings Schemas
+	// 5. Register Settings Schemas
 	ctx.Settings().Register(extpoints.SettingSchema{
 		Key:         "user.registration_enabled",
 		Default:     true,