panel-notifications: schema and store

Migration 0004_notifications.sql, DDL only: users.notify, a user's own
switch, on by default; notify_state, one row per subject the panel has
written about (dns:<domain>, cert, queue, limit:<domain>:<login>), so that
each thing is said once and a restart says nothing again.

Store: the instance switch as a settings key, on unless turned off;
SetUserNotify; NotifyRecipients - global users for everything, domain users
for the domains on their list or all of them, only with an e-mail and their
switch on, a shared mailbox once; the state's get, set, delete and list by
prefix; LimitRefusalsSince, the mail refused under a level-2 limit per domain
and application.

Nothing is sent yet. Checked on Windows: go vet clean, go test
./internal/store passes with the schema test at user_version 4. The page
guards fail from the design commit 87fa0a4 until the pages are built.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -5,6 +5,15 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 
 ## [Unreleased]
 
+### Added
+
+- **Schema for notifications** (migration `0004_notifications.sql`, DDL only):
+  a user's own switch, `users.notify`, on for everyone; `notify_state`, what
+  the panel has already written to its users about. The instance switch is a
+  settings key, on unless turned off
+  ([plans/panel-notifications.md](docs/plans/panel-notifications.md)). Nothing
+  is sent yet.
+
 ## [2.2.0] - 2026-10-12
 
 ### Added