fix(server): do not reset fileCreatedDate

When marking an offline asset as online again, do not reset the
fileCreatedAt value. This value contains the "true" date, copied
from exif.dateTimeOriginal. If we overwrite this value, we'd need
to run the metadata extraction job again. Instead, we just leave
the old (and correct) value in place.

fixes #15640

server/src/services/library.service.spec.ts

@@ -343,6 +343,27 @@ describe(LibraryService.name, () => {
         originalFileName: 'path.jpg',
       });
     });
+
+    // regression test for https://github.com/immich-app/immich/issues/15640
+    it('should not touch fileCreatedAt when un-trashing an asset previously marked as offline', async () => {
+      // After the initial import, fileCreatedAt is overwritten with metadata information (dateTimeOriginal).
+      // We need to retain this information. If we overwrite this, we'd need to trigger a new metadata extraction job.
+
+      const mockAssetJob: ILibraryAssetJob = {
+        id: assetStub.external.id,
+        importPaths: ['/'],
+        exclusionPatterns: [],
+      };
+
+      assetMock.getById.mockResolvedValue(assetStub.trashedOffline);
+      storageMock.stat.mockResolvedValue({ mtime: assetStub.trashedOffline.fileModifiedAt } as Stats);
+
+      await expect(sut.handleSyncAsset(mockAssetJob)).resolves.toBe(JobStatus.SUCCESS);
+
+      expect(assetMock.updateAll).toHaveBeenCalledWith([assetStub.trashedOffline.id], expect.not.objectContaining({
+        "fileCreatedAt": expect.anything(),
+      }));
+    });
   });
 
   it('should update file when mtime has changed', async () => {