reducing memory usage on OU TDE notebook

[jkrick]

This PR reduces memory usage in the TDE light curve notebook to address CI failures likely caused by memory constraints. It does not effect the science or goals of the notebook

Changes:

• Replace full-image background estimation (np.nanmedian/np.nanstd over 4088×4088 pixels) with a local annulus-based background using photutils SkyCircularAnnulus and ApertureStats
• Add del data in show_gallery() to free each full image array immediately after display
• Add del img in make_cutout() to free the full image array after the cutout is extracted
• Reduce default number of gallery images from 6 to 4
• Reduce default number of cutout images from 6 to 4

[jkrick]

maybe this closes #317 ?