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/routes/admin/integrations/+page.svelte

@@ -30,6 +30,7 @@
   import AcoustIDCard from '$lib/components/AcoustIDCard.svelte';
   import NetworkSettingsCard from '$lib/components/NetworkSettingsCard.svelte';
   import PublicAddressCard from '$lib/components/PublicAddressCard.svelte';
+  import NotificationEmailCard from '$lib/components/NotificationEmailCard.svelte';
   import type { LidarrConfig, LidarrTestResult } from '$lib/api/types';
 
   // Lidarr connection panel. The "saved api key" is masked as "***" on GET —
@@ -754,7 +755,7 @@
     <div>
       <h3 class="font-display text-lg font-medium text-text-primary">Email (SMTP)</h3>
       <p class="mt-1 text-sm text-text-secondary">
-        Configure outgoing email for password resets. The "Send test email"
+        Configure outgoing email for password resets and notifications. The "Send test email"
         button verifies your config end-to-end.
       </p>
     </div>
@@ -811,6 +812,8 @@
     </div>
   </section>
 
+  <NotificationEmailCard />
+
   <!-- Client IP detection. Belongs here rather than in user Settings: it
        describes how Minstrel sits behind other infrastructure, same as every
        other card on this page, and it's an operator-wide setting rather than