fix(scanner): preserve created_at when moving songs between libraries (#5055)

* fix: preserve created_at when moving songs between libraries (#5050)

When songs are moved between libraries, their creation date was being
reset to the current time, causing them to incorrectly appear in
"Recently Added". Three changes fix this:

1. Add hash:"ignore" to AlbumID in MediaFile struct so that Equals()
   works for cross-library moves (AlbumID includes library prefix,
   making hashes always differ between libraries)

2. Preserve album created_at in moveMatched() via CopyAttributes,
   matching the pattern already used in persistAlbum() for
   within-library album ID changes

3. Only set CreatedAt in Put() when it's zero (new files), and
   explicitly copy missing.CreatedAt to the target in moveMatched()
   as defense-in-depth for the INSERT code path

* test: add regression tests for created_at preservation (#5050)

Add tests covering the three aspects of the fix:
- Scanner: moveMatched preserves missing track's created_at
- Scanner: CopyAttributes called for album created_at on album change
- Scanner: CopyAttributes not called when album ID stays the same
- Persistence: Put sets CreatedAt to now for new files with zero value
- Persistence: Put preserves non-zero CreatedAt on insert
- Persistence: Put does not reset CreatedAt on update

Also adds CopyAttributes to MockAlbumRepo for test support.

* test: verify album created_at is updated in cross-library move test (#5050)

Added end-to-end assertion in the cross-library move test to verify that
the new album's CreatedAt field is actually set to the original value after
CopyAttributes runs, not just that the method was called. This strengthens
the test by confirming the mock correctly propagates the timestamp.

persistence/mediafile_repository.go

@@ -148,7 +148,9 @@ func (r *mediaFileRepository) Exists(id string) (bool, error) {
 }
 
 func (r *mediaFileRepository) Put(m *model.MediaFile) error {
-	m.CreatedAt = time.Now()
+	if m.CreatedAt.IsZero() {
+		m.CreatedAt = time.Now()
+	}
 	id, err := r.putByMatch(Eq{"path": m.Path, "library_id": m.LibraryID}, m.ID, &dbMediaFile{MediaFile: m})
 	if err != nil {
 		return err

persistence/mediafile_repository_test.go

@@ -104,6 +104,68 @@ var _ = Describe("MediaRepository", func() {
 		}
 	})
 
+	Describe("Put CreatedAt behavior (#5050)", func() {
+		It("sets CreatedAt to now when inserting a new file with zero CreatedAt", func() {
+			before := time.Now().Add(-time.Second)
+			newFile := model.MediaFile{ID: id.NewRandom(), LibraryID: 1, Path: "/test/created-at-zero.mp3"}
+			Expect(mr.Put(&newFile)).To(Succeed())
+
+			retrieved, err := mr.Get(newFile.ID)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(retrieved.CreatedAt).To(BeTemporally(">", before))
+
+			_ = mr.Delete(newFile.ID)
+		})
+
+		It("preserves CreatedAt when inserting a new file with non-zero CreatedAt", func() {
+			originalTime := time.Date(2020, 3, 15, 10, 30, 0, 0, time.UTC)
+			newFile := model.MediaFile{
+				ID:        id.NewRandom(),
+				LibraryID: 1,
+				Path:      "/test/created-at-preserved.mp3",
+				CreatedAt: originalTime,
+			}
+			Expect(mr.Put(&newFile)).To(Succeed())
+
+			retrieved, err := mr.Get(newFile.ID)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(retrieved.CreatedAt).To(BeTemporally("~", originalTime, time.Second))
+
+			_ = mr.Delete(newFile.ID)
+		})
+
+		It("does not reset CreatedAt when updating an existing file", func() {
+			originalTime := time.Date(2019, 6, 1, 12, 0, 0, 0, time.UTC)
+			fileID := id.NewRandom()
+			newFile := model.MediaFile{
+				ID:        fileID,
+				LibraryID: 1,
+				Path:      "/test/created-at-update.mp3",
+				Title:     "Original Title",
+				CreatedAt: originalTime,
+			}
+			Expect(mr.Put(&newFile)).To(Succeed())
+
+			// Update the file with a new title but zero CreatedAt
+			updatedFile := model.MediaFile{
+				ID:        fileID,
+				LibraryID: 1,
+				Path:      "/test/created-at-update.mp3",
+				Title:     "Updated Title",
+				// CreatedAt is zero - should NOT overwrite the stored value
+			}
+			Expect(mr.Put(&updatedFile)).To(Succeed())
+
+			retrieved, err := mr.Get(fileID)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(retrieved.Title).To(Equal("Updated Title"))
+			// CreatedAt should still be the original time (not reset)
+			Expect(retrieved.CreatedAt).To(BeTemporally("~", originalTime, time.Second))
+
+			_ = mr.Delete(fileID)
+		})
+	})
+
 	It("checks existence of mediafiles in the DB", func() {
 		Expect(mr.Exists(songAntenna.ID)).To(BeTrue())
 		Expect(mr.Exists("666")).To(BeFalse())