feat(notifications): grouped email digest, new music as a daily summary (#5346)

Nothing is emailed per event. New music (request_completed) goes out at
most once a day, at the summary hour in each user's own timezone, grouped
by artist. Everything else is batched: one email a window after the first
un-emailed item, holding whatever accumulated.

- Migration 0074: notification_email_settings (summary hour, batch window,
  admin-configurable) and user_notification_email_state (batch start, last
  sent, failures and retry_after per user and group). Existing rows are
  stamped emailed so the upgrade sends no backlog.
- The Notifier stamps emailed_at at write time when the recipient's email
  channel is off, so turning email on later doesn't send old items.
- Read rows are never selected. A row is stamped only after the mailer
  accepts, in one transaction with the state, against the read's clock, so
  a coalesced row updated mid-send stays pending.
- A failed send backs off 5m doubling to 6h; SMTP not configured just waits.
- Links come from the public address; without one the email has none.
- The mailer now RFC 2047-encodes subjects and strips line breaks from them.
- Admin → Integrations gains a Notification emails card.

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

web/src/lib/api/admin.ts

@@ -921,3 +921,19 @@ export async function updateReacquisitionSettings(
 ): Promise<ReacquisitionSettings> {
   return api.put<ReacquisitionSettings>('/api/admin/library/reacquisition', s);
 }
+
+// Notification emails (M489 #5346): when the grouped emails go out.
+export type NotificationEmailSettings = {
+  summary_hour: number;
+  batch_window_minutes: number;
+};
+
+export async function getNotificationEmailSettings(): Promise<NotificationEmailSettings> {
+  return api.get<NotificationEmailSettings>('/api/admin/notification-email');
+}
+
+export async function updateNotificationEmailSettings(
+  s: NotificationEmailSettings
+): Promise<NotificationEmailSettings> {
+  return api.put<NotificationEmailSettings>('/api/admin/notification-email', s);
+}