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>

internal/lidarrrequests/service.go

@@ -141,6 +141,18 @@ func DisplayName(row dbq.LidarrRequest) string {
 	return row.ArtistName
 }
 
+// requestedTitle is the album or track a request names, or "" for an
+// artist request. DisplayName is the artist and this, joined.
+func requestedTitle(row dbq.LidarrRequest) string {
+	switch {
+	case row.Kind == dbq.LidarrRequestKindTrack && row.TrackTitle != nil:
+		return *row.TrackTitle
+	case row.Kind == dbq.LidarrRequestKindAlbum && row.AlbumTitle != nil:
+		return *row.AlbumTitle
+	}
+	return ""
+}
+
 func validateKindFields(p CreateParams) error {
 	if p.LidarrArtistMBID == "" || p.ArtistName == "" {
 		return fmt.Errorf("%w: artist_mbid and artist_name are always required", ErrInvalidKindFields)