fix(escalations): atomic claim + self-claim rejection + queue exclusion

Codex review pass on the escalation wedge. Reworks claim_session from
read-then-write to a conditional UPDATE so two seniors racing can't both
win, blocks the original engineer from claiming their own handoff, and
filters self-escalated sessions out of the dashboard escalation queue.
Also preassigns the handoff UUID before flush so the compatibility
escalation_package payload carries it. Removes legacy frontend pickup
state (claiming, handleStartHere) that broke tsc --noEmit.

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

backend/app/api/endpoints/ai_sessions.py

@@ -689,6 +689,7 @@ async def get_escalation_queue(
         .where(
             scope_filter,
             AISession.status.in_(("requesting_escalation", "escalated")),
+            AISession.user_id != current_user.id,
         )
         .order_by(AISession.created_at.desc())
     )

backend/app/api/endpoints/session_handoffs.py

@@ -144,6 +144,8 @@ async def claim_handoff(
                 "claimed_at": e.claimed_at.isoformat(),
             },
         )
+    except PermissionError as e:
+        raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=str(e))
     except ValueError as e:
         raise HTTPException(status_code=404, detail=str(e))