Fix silent deserialization failure with unique together constraint

[ozer550]

Summary

• Added IntegrityError to the caught exceptions in the deserialization pipeline so unique constraint violations are handled instead of silently ignored.
• Replaced the bulk upsert in the non-self-referential branch with per-record saves.
• Added regression tests in test_controller.py for the unique constraint scenario.

TODO

• Have tests been written for the new code?
• Has documentation been written/updated?
• New dependencies (if any) added to requirements file

Reviewer guidance

• Check if the regression tests make sense!
• Manually syncing two devices with voilating unique constraint for fields.

Issues addressed

closes #306

[ozer550]

This is so that the outer transaction stays healthy, and we can keep saving other records.

[bjester]

This is really mostly complete, just some cleanup and tidying code.

And the small deserialization_exception enhancement!

[bjester]

Looks great! The TDD driven approach worked really well here. Thanks for tackling this @ozer550