fix(l1): add missing next-node + escalations routes; reconcile Phase-1 intake tests

An earlier anchor-edit silently failed, so POST /sessions/{id}/next-node and
GET /escalations were never added (they 404'd). Add both, anchored on the real
/escalate-without-walk route.

Phase-1 test_l1_endpoints tests used POST /intake to create adhoc setup sessions,
but Phase 2A intake now dispatches via match_or_build (build/matched/suggest/
out_of_scope — never adhoc). Add a _create_adhoc_session service helper and route
the step/notes/resolve/escalate/cross-account setup through it; rewrite
test_intake_adhoc as test_intake_build_creates_ai_build_session (mocked outcome).

All green: test_l1_endpoints + test_l1_api_ai_build = 25 passed; full Phase 2A
backend service/unit/model suite = 56 passed; notification suite = 18 passed.

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

backend/app/api/endpoints/l1.py

@@ -284,6 +284,68 @@ async def post_escalate(
     return _to_response(updated)
 
 
+@router.post("/sessions/{session_id}/next-node", response_model=NextNodeResponse)
+async def next_node(
+    session_id: UUID,
+    payload: NextNodeRequest,
+    db: Annotated[AsyncSession, Depends(get_db)],
+    user: Annotated[User, Depends(require_l1_or_coverage)],
+):
+    """Record the answer/ack on the current node, then generate the next node.
+
+    problem_text comes from the linked internal ticket; category from the hidden
+    meta entry seeded at intake (ai_tree_builder skips meta entries). node_text is
+    the rendered text of the node being answered (the client holds it) so the
+    walked path and the captured tree stay legible.
+    """
+    session = await _get_session_or_404(db, session_id, user)
+    ticket = await internal_ticket_service.get_ticket(
+        db, ticket_id=UUID(session.ticket_id)
+    )
+    problem_text = ticket.problem_statement if ticket else ""
+    category = next(
+        (s.get("category") for s in (session.walked_path or [])
+         if s.get("node_type") == "meta"),
+        "unknown",
+    )
+    try:
+        node = await l1_session_service.advance_ai_build(
+            db,
+            session_id=session_id,
+            problem_text=problem_text,
+            category=category or "unknown",
+            node_id=payload.node_id,
+            node_text=payload.node_text,
+            answer=payload.answer,
+            note=payload.note,
+        )
+    except ValueError as exc:
+        raise HTTPException(
+            status_code=http_status.HTTP_409_CONFLICT, detail=str(exc)
+        )
+    await db.commit()
+    return NextNodeResponse(node=node, session_status=session.status)
+
+
+@router.get("/escalations", response_model=list[WalkSessionResponse])
+async def l1_escalations(
+    db: Annotated[AsyncSession, Depends(get_db)],
+    user: Annotated[User, Depends(require_engineer_or_admin)],
+    limit: int = 50,
+):
+    """Engineer-visible list of escalated L1 sessions (the handoff queue)."""
+    rows = await db.execute(
+        select(L1WalkSession)
+        .where(
+            L1WalkSession.account_id == user.account_id,
+            L1WalkSession.status == "escalated",
+        )
+        .order_by(L1WalkSession.last_step_at.desc())
+        .limit(limit)
+    )
+    return [_to_response(s) for s in rows.scalars()]
+
+
 @router.post("/escalate-without-walk", response_model=WalkSessionResponse)
 async def post_escalate_without_walk(
     payload: EscalateWithoutWalkRequest,