feat: add handoff API endpoints with queue and integration tests

Four endpoints: create handoff (park/escalate), list handoff history,
claim session, and team queue. Two routers: session-scoped router with
prefix /ai-sessions/{session_id} and queue_router with prefix /ai-sessions.
queue_router registered before ai_sessions.router to avoid /{session_id}
path conflict on GET /ai-sessions/queue.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/tests/test_session_handoffs_api.py

@@ -0,0 +1,60 @@
+"""API endpoint tests for session handoffs."""
+import pytest
+from httpx import AsyncClient
+
+from app.models.ai_session import AISession
+
+
+@pytest.mark.asyncio
+async def test_create_park_handoff_api(client: AsyncClient, test_user, auth_headers, test_db):
+    """POST /ai-sessions/{id}/handoff with intent=park."""
+    session = AISession(
+        user_id=test_user["user_data"]["id"],
+        account_id=test_user["user_data"]["account_id"],
+        session_type="guided",
+        intake_type="free_text",
+        intake_content={"text": "test"},
+        status="active",
+        confidence_tier="discovery",
+        conversation_messages=[],
+    )
+    test_db.add(session)
+    await test_db.commit()
+
+    resp = await client.post(
+        f"/api/v1/ai-sessions/{session.id}/handoff",
+        headers=auth_headers,
+        json={"intent": "park", "engineer_notes": "Waiting for logs"},
+    )
+    assert resp.status_code == 201
+    data = resp.json()
+    assert data["intent"] == "park"
+
+
+@pytest.mark.asyncio
+async def test_get_queue(client: AsyncClient, test_user, auth_headers, test_db):
+    """GET /ai-sessions/queue returns unclaimed handoffs."""
+    session = AISession(
+        user_id=test_user["user_data"]["id"],
+        account_id=test_user["user_data"]["account_id"],
+        session_type="guided",
+        intake_type="free_text",
+        intake_content={"text": "test"},
+        status="active",
+        confidence_tier="discovery",
+        conversation_messages=[],
+    )
+    test_db.add(session)
+    await test_db.commit()
+
+    # Create a handoff
+    await client.post(
+        f"/api/v1/ai-sessions/{session.id}/handoff",
+        headers=auth_headers,
+        json={"intent": "escalate", "engineer_notes": "Need help"},
+    )
+
+    resp = await client.get("/api/v1/ai-sessions/queue", headers=auth_headers)
+    assert resp.status_code == 200
+    data = resp.json()
+    assert len(data) >= 1