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/notifications/notifier.go

@@ -92,7 +92,7 @@ func (n *Notifier) Notify(ctx context.Context, kind Kind, to Recipients, payload
 		if !channels[id].Inbox {
 			continue
 		}
-		if err := write(ctx, q, kind, id, body); err != nil {
+		if err := write(ctx, q, kind, id, body, channels[id].Email); err != nil {
 			return fmt.Errorf("notifications: write %s: %w", kind, err)
 		}
 		delivered = append(delivered, id)
@@ -166,11 +166,14 @@ func channelsFor(ctx context.Context, q *dbq.Queries, kind Kind, ids []pgtype.UU
 	return out, nil
 }
 
-func write(ctx context.Context, q *dbq.Queries, kind Kind, userID pgtype.UUID, body []byte) error {
+// write stores one row. emailWanted is the recipient's email channel for the
+// kind: a row nobody wants emailed is stamped emailed_at at once, so the
+// digest never sends it, even if email is switched on later.
+func write(ctx context.Context, q *dbq.Queries, kind Kind, userID pgtype.UUID, body []byte, emailWanted bool) error {
 	key := kind.coalesceKey()
 	if key == "" {
 		_, err := q.InsertNotification(ctx, dbq.InsertNotificationParams{
-			UserID: userID, Kind: string(kind), Payload: body,
+			UserID: userID, Kind: string(kind), Payload: body, EmailWanted: emailWanted,
 		})
 		return err
 	}
@@ -179,6 +182,7 @@ func write(ctx context.Context, q *dbq.Queries, kind Kind, userID pgtype.UUID, b
 		Kind:        string(kind),
 		Payload:     body,
 		CoalesceKey: &key,
+		EmailWanted: emailWanted,
 		SumCount:    specs[kind].sumCount,
 	})
 	return err