Separate photoscan registrations from transducer-tracking results

[ebrahimebrahim]

Transducer-tracking results currently embed a photoscan-to-volume transform and approval state, which duplicates registration data and cannot cleanly represent multiple registration attempts.

• Add a PhotoscanRegistration model with a stable session-local ID.
• Have tracking results refer to the selected registration by ID.
• Migrate legacy session data with embedded transforms.
• Store photoscan and photocollection membership explicitly on sessions.

[ebrahimebrahim]

(this issue was created post hoc, to be addressed in #478)