feat(server): add index-based play queue endpoints to native API (#4210)

* Add migration converting playqueue current to index

* refactor

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(queue): ensure valid current index and improve test coverage

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

tests/mock_playqueue_repo.go

@@ -0,0 +1,39 @@
+package tests
+
+import (
+	"errors"
+
+	"github.com/navidrome/navidrome/model"
+)
+
+type MockPlayQueueRepo struct {
+	model.PlayQueueRepository
+	Queue *model.PlayQueue
+	Err   bool
+}
+
+func (m *MockPlayQueueRepo) Store(q *model.PlayQueue) error {
+	if m.Err {
+		return errors.New("error")
+	}
+	copyItems := make(model.MediaFiles, len(q.Items))
+	copy(copyItems, q.Items)
+	qCopy := *q
+	qCopy.Items = copyItems
+	m.Queue = &qCopy
+	return nil
+}
+
+func (m *MockPlayQueueRepo) Retrieve(userId string) (*model.PlayQueue, error) {
+	if m.Err {
+		return nil, errors.New("error")
+	}
+	if m.Queue == nil || m.Queue.UserID != userId {
+		return nil, model.ErrNotFound
+	}
+	copyItems := make(model.MediaFiles, len(m.Queue.Items))
+	copy(copyItems, m.Queue.Items)
+	qCopy := *m.Queue
+	qCopy.Items = copyItems
+	return &qCopy, nil
+}