Make max_time_gap a configurable Project setting

[mohamedelabbas1996]

Summary

Currently, the grouping of images into sessions uses a hardcoded max_time_gap value (defaults to 120 minutes) to determine session breaks. However, different projects may have different monitoring schedules or needs. For example, some may want to split sessions more aggressively, others less so.

This issue proposes moving the max_time_gap value into the Project model so each project can define its own session grouping logic. This aligns with ongoing work in #893 to make filters configurable per project.

[mihow]

I added a field for this in #918, but the value is not used anywhere yet. I think we can bring #904 up-to-date with main and then use the new project setting there.