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/db/dbq/models.go

@@ -472,6 +472,13 @@ type NetworkSetting struct {
 	PublicUrl        string
 }
 
+type NotificationEmailSetting struct {
+	ID                 bool
+	SummaryHour        int32
+	BatchWindowMinutes int32
+	UpdatedAt          pgtype.Timestamptz
+}
+
 type PasswordReset struct {
 	Token     string
 	UserID    pgtype.UUID
@@ -842,6 +849,15 @@ type UserNotification struct {
 	EmailedAt   pgtype.Timestamptz
 }
 
+type UserNotificationEmailState struct {
+	UserID        pgtype.UUID
+	EmailGroup    string
+	BatchOpenedAt pgtype.Timestamptz
+	LastSentAt    pgtype.Timestamptz
+	Failures      int32
+	RetryAfter    pgtype.Timestamptz
+}
+
 type UserNotificationPref struct {
 	UserID    pgtype.UUID
 	Kind      string