Expose session regrouping as a job stage when syncing images

[mihow]

Summary

When syncing captures from storage, the session regrouping step is buried inside Deployment.save() and not visible as a job stage. Users have no visibility into how many sessions were created/updated, or whether captures were left ungrouped (e.g., due to missing timestamps).

Current behavior

• sync_captures() calls self.save(regroup_async=False) which triggers group_images_into_events() internally
• The job logs show "Saving and recalculating sessions for deployment" as an INFO message but it's not a tracked stage
• If images have NULL timestamps, they silently remain ungrouped with no warning in the job

Desired behavior

• Add a "Recalculate sessions" stage to the sync job progress
• Show number of sessions created/updated as a stage parameter
• Warn if any captures are not assigned to a session (e.g., ungrouped_captures param)
• Log a warning with the count of ungrouped images

Related

• PR Fix incorrect session grouping  #904 refactors group_images_into_events and may be a good place to implement this
• Discovered while debugging Farmscape NSCF camera imports where 2-digit year timestamps caused NULL timestamps on all 183 captures

Key code locations

• ami/main/models.py:782-851 — sync_captures() method
• ami/main/models.py:979-993 — Deployment.save() regroup logic
• ami/main/models.py:1290-1372 — group_images_into_events() function
• ami/jobs/models.py:596-640 — DataStorageSyncJob job type