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/dbtest/reset.go

@@ -101,6 +101,7 @@ var dataTables = []string{
 	// never deleted, so rows a test wrote for it would otherwise survive.
 	"user_notifications",
 	"user_notification_prefs",
+	"user_notification_email_state", // #5346
 	"tracks",
 	"albums",
 	"artists",
@@ -158,4 +159,11 @@ func ResetDB(t *testing.T, pool *pgxpool.Pool) {
 	); err != nil {
 		t.Fatalf("dbtest.ResetDB reset loudness settings: %v", err)
 	}
+	// Notification email settings (M489 #5346), reset the same way.
+	if _, err := pool.Exec(ctx, `
+		UPDATE notification_email_settings
+		   SET summary_hour = DEFAULT, batch_window_minutes = DEFAULT, updated_at = DEFAULT`,
+	); err != nil {
+		t.Fatalf("dbtest.ResetDB reset notification email settings: %v", err)
+	}
 }