fix(scanner): always refresh folder image time when adding first image (#3764)

* fix(scanner): Always refresh folder image time when adding first image

Currently, the `images_updated_at` field is only set to the image modification time.
However, in cases where a new image is added _and_ said image is older than the folder mod time, the field is not updated properly.

In this the case where `images_updated_at` is null (no images were ever added) and a new images is found, use the folder modification time instead of image modification time.

**Note**, this doesn't handle cases such as replacing a newer image with an older one.

* simplify image update at

* we don't want to set imagesUpdatedAt when there's no images in the folder

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

---------

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

utils/time.go

@@ -0,0 +1,13 @@
+package utils
+
+import "time"
+
+func TimeNewest(times ...time.Time) time.Time {
+	newest := time.Time{}
+	for _, t := range times {
+		if t.After(newest) {
+			newest = t
+		}
+	}
+	return newest
+}