fix(escalations): bell-icon notification opens the pickup flow

Two backend changes that unbreak the senior-pickup path from the
notification panel:

1. notification_service: session.escalated link template now ends with
   ?pickup=true so the senior lands in the handoff/pickup flow on
   click. Without it, navigation hit /pilot/:id directly, which then
   404'd on the GET because the senior isn't yet escalated_to_id —
   the user perceives this as the bell-icon "just clearing the
   notification".

2. ai_sessions GET access: any account member can now read an escalated
   session's detail when status is requesting_escalation or escalated.
   The owner-only guard was overly restrictive for explicitly-shared
   in-transit states. Tenant boundary is enforced by RLS on the
   underlying query, so account-scope is the right ceiling here. After
   pickup, the existing handler/escalated_to_id checks still apply.

Verified live: re-login as the senior engineer and GET the active
escalated session — now returns 200 with full detail. Focused test
subset plus tests/test_sessions.py and tests/test_session_sharing.py
→ 94 passed in 43.26s, no regressions.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/api/endpoints/ai_sessions.py

@@ -901,10 +901,21 @@ async def get_session(
     if not session:
         raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Session not found")
 
-    # Allow access if user is owner, escalation target, or picked-up handler
+    # Allow access if user is owner, escalation target, or picked-up handler.
+    # Sessions in transit (requesting_escalation / escalated) are also
+    # readable by any account member — the whole point of escalation is that
+    # other techs can see the context before claiming. Tenant boundary is
+    # enforced by RLS on the underlying query, so account-scope is the right
+    # ceiling for in-transit reads.
     pkg = session.escalation_package or {}
     is_handler = pkg.get("picked_up_by") == str(current_user.id)
-    if session.user_id != current_user.id and session.escalated_to_id != current_user.id and not is_handler:
+    is_in_transit = session.status in ("requesting_escalation", "escalated")
+    if (
+        session.user_id != current_user.id
+        and session.escalated_to_id != current_user.id
+        and not is_handler
+        and not is_in_transit
+    ):
         raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Session not found")
 
     return _build_session_detail(session)