Add transducer_transform_source_id to SolutionInfo for source-liveness tracking

Add transducer_transform_source_id: str | None field to
openlifu.db.session.SolutionInfo so downstream consumers
(SlicerOpenLIFU#628 followup) can distinguish "the specific VF or TT result
that produced this solution" from the coarser
transducer_transform_source ("virtual_fit" vs. "localization") kind.

Motivation

After the array_transform work in openlifu-python#491, solutions render at
their compute-time pose regardless of subsequent approval churn. But there
was no way to tell whether the specific VF / TT result that produced the
solution is still (a) present in the session, (b) still approved. Downstream
UI needs this to badge solutions with a status column (Live / Revoked /
Missing / Legacy) and warn / block sending stale solutions to hardware.

Field

• transducer_transform_source_id: str | None (default None for legacy
  compat).
• Opaque identifier -- interpretation is source-dependent, delegated to the
  downstream app. SlicerOpenLIFU will use "<target_id>:<rank>" for VF
  results (composite key that stays stable across approval-only changes)
  and the TransducerTrackingResult.id for TT results.
• Sessions saved before this field existed round-trip as None;
  downstream consumers should degrade gracefully (treat as "legacy" status
  rather than "missing").

Round-trip tests added.

Downstream

Consumed by SlicerOpenLIFU#628 followup: Solutions-table source-status
column, hardware-send guard, "delete solutions with revoked source" action.

Start in openlifu.db.session at SolutionInfo and inspect its existing serialization and deserialization paths. Add the optional source identifier with a None default, then run or extend the round-trip tests to verify new values persist and pre-existing sessions load as None.