Guard genlocke data integrity edge cases

Block deletion of runs linked to a genlocke leg, prevent reactivating
completed/failed genlocke-linked runs, and guard encounter deletion
against genlocke transfer references with clear 400 errors.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/app/api/encounters.py

@@ -159,6 +159,19 @@ async def delete_encounter(
     if encounter is None:
         raise HTTPException(status_code=404, detail="Encounter not found")
 
+    # Block deletion if encounter is referenced by a genlocke transfer
+    transfer_result = await session.execute(
+        select(GenlockeTransfer.id).where(
+            (GenlockeTransfer.source_encounter_id == encounter_id)
+            | (GenlockeTransfer.target_encounter_id == encounter_id)
+        )
+    )
+    if transfer_result.scalar_one_or_none() is not None:
+        raise HTTPException(
+            status_code=400,
+            detail="Cannot delete an encounter that is part of a genlocke transfer.",
+        )
+
     await session.delete(encounter)
     await session.commit()
     return Response(status_code=204)