fix: harden playback and migrations

internal/features/playback/service_utils.go

@@ -0,0 +1,23 @@
+package playback
+
+import (
+	"strings"
+)
+
+func toSubtitleItems(source StreamSource) []SubtitleItem {
+	items := make([]SubtitleItem, 0, len(source.Subtitles))
+	for _, subtitle := range source.Subtitles {
+		targetURL := strings.TrimSpace(subtitle.URL)
+		if targetURL == "" {
+			continue
+		}
+
+		items = append(items, SubtitleItem{
+			Lang:    strings.TrimSpace(subtitle.Lang),
+			URL:     targetURL,
+			Referer: source.Referer,
+		})
+	}
+
+	return items
+}