refactor(server): steward 决策链路改用 LangGraph 编排

- 新增 StewardGraphPlannerService，用 LangGraph 状态图编排意图识别→流程判断→模型/规则分支→兜底，替代原 planner 内线性调用
- 新增 StewardGraphRuntimeService 编排运行时决策与槽位决策；StewardActionContracts/Executor 统一动作合约与执行
- steward_intent_agent/application_fact_resolver/runtime_chat 适配图执行器，config 暴露图相关开关
- pyproject/uv.lock 新增 langgraph 依赖
- 新增 graph_planner/graph_runtime/action_executor 测试，更新 intent_agent/planner/fact_resolver/runtime_chat/reimbursement 测试

server/src/app/api/v1/endpoints/steward.py

@@ -10,10 +10,13 @@ from fastapi.responses import StreamingResponse
 from sqlalchemy import select
 from sqlalchemy.orm import Session
 
-from app.api.deps import get_db
+from app.api.deps import CurrentUserContext, get_current_user, get_db
+from app.core.config import get_settings
 from app.models.financial_record import ExpenseClaim
 from app.schemas.common import ErrorResponse
 from app.schemas.steward import (
+    StewardActionExecuteRequest,
+    StewardActionExecuteResponse,
     StewardPlanRequest,
     StewardPlanResponse,
     StewardRuntimeDecisionRequest,
@@ -27,6 +30,9 @@ from app.services.expense_claim_draft_flow import APPROVED_APPLICATION_LINK_STAT
 from app.services.expense_claims import ExpenseClaimService
 from app.services.runtime_chat import RuntimeChatService
 from app.services.steward_flow_state import StewardFlowStateService
+from app.services.steward_graph_action_runtime import StewardGraphActionRuntime
+from app.services.steward_graph_planner import StewardGraphPlannerService
+from app.services.steward_graph_runtime import StewardGraphRuntime
 from app.services.steward_intent_agent import StewardIntentAgent
 from app.services.steward_off_topic_agent import StewardOffTopicAgent
 from app.services.steward_planner import StewardPlannerService
@@ -35,6 +41,8 @@ from app.services.steward_slot_decision_agent import StewardSlotDecisionAgent
 
 router = APIRouter(prefix="/steward")
 DbSession = Annotated[Session, Depends(get_db)]
+CurrentUser = Annotated[CurrentUserContext, Depends(get_current_user)]
+StewardPlannerLike = StewardPlannerService | StewardGraphPlannerService
 
 
 @router.post(
@@ -69,7 +77,7 @@ def create_steward_slot_decision(
     payload: StewardSlotDecisionRequest,
     db: DbSession,
 ) -> StewardSlotDecisionResponse:
-    return StewardSlotDecisionAgent(RuntimeChatService(db)).decide(payload)
+    return _decide_steward_slot(payload, RuntimeChatService(db))
 
 
 @router.post(
@@ -83,10 +91,27 @@ def create_steward_runtime_decision(
     db: DbSession,
 ) -> StewardRuntimeDecisionResponse:
     hydrated_payload = _hydrate_runtime_decision_payload(db, payload)
-    decision = StewardRuntimeDecisionAgent(RuntimeChatService(db)).decide(hydrated_payload)
+    decision = _decide_steward_runtime(hydrated_payload, RuntimeChatService(db))
     return _attach_runtime_conversation_state(db, hydrated_payload, decision)
 
 
+@router.post(
+    "/actions/execute",
+    response_model=StewardActionExecuteResponse,
+    summary="执行小财管家白名单动作",
+    description=(
+        "按 LangGraph 规划出的 action step 执行确定性业务动作；"
+        "当 action 未知、缺字段、缺确认或预检查未通过时直接阻断并返回结构化兜底结果。"
+    ),
+)
+def execute_steward_action(
+    payload: StewardActionExecuteRequest,
+    db: DbSession,
+    current_user: CurrentUser,
+) -> StewardActionExecuteResponse:
+    return StewardGraphActionRuntime(db).execute(payload, current_user)
+
+
 @router.post(
     "/plans/stream",
     summary="流式生成小财管家任务计划",
@@ -101,7 +126,7 @@ async def stream_steward_plan(payload: StewardPlanRequest, db: DbSession) -> Str
 
 async def _iter_steward_plan_events(
     payload: StewardPlanRequest,
-    planner: StewardPlannerService,
+    planner: StewardPlannerLike,
     db: Session,
 ) -> AsyncIterator[str]:
     yield _encode_stream_event(
@@ -135,18 +160,53 @@ def _encode_stream_event(event: str, data: dict[str, Any]) -> str:
     return json.dumps({"event": event, "data": data}, ensure_ascii=False) + "\n"
 
 
-def _build_steward_planner(db: Session) -> StewardPlannerService:
+def _build_steward_planner(db: Session) -> StewardPlannerLike:
     runtime_chat = RuntimeChatService(db)
+    if get_settings().steward_agent_runtime.strip().lower() == "langgraph":
+        return StewardGraphPlannerService(
+            intent_agent=StewardIntentAgent(runtime_chat),
+            off_topic_agent=StewardOffTopicAgent(runtime_chat),
+        )
     return StewardPlannerService(
         intent_agent=StewardIntentAgent(runtime_chat),
         off_topic_agent=StewardOffTopicAgent(runtime_chat),
     )
 
 
+def _decide_steward_slot(
+    payload: StewardSlotDecisionRequest,
+    runtime_chat: Any,
+) -> StewardSlotDecisionResponse:
+    legacy_agent = StewardSlotDecisionAgent(runtime_chat)
+    if not _should_use_langgraph_runtime():
+        return legacy_agent.decide(payload)
+    try:
+        return StewardGraphRuntime(runtime_chat).decide_slot(payload)
+    except Exception:
+        return legacy_agent.decide(payload)
+
+
+def _decide_steward_runtime(
+    payload: StewardRuntimeDecisionRequest,
+    runtime_chat: Any,
+) -> StewardRuntimeDecisionResponse:
+    legacy_agent = StewardRuntimeDecisionAgent(runtime_chat)
+    if not _should_use_langgraph_runtime():
+        return legacy_agent.decide(payload)
+    try:
+        return StewardGraphRuntime(runtime_chat).decide_runtime(payload)
+    except Exception:
+        return legacy_agent.decide(payload)
+
+
+def _should_use_langgraph_runtime() -> bool:
+    return get_settings().steward_agent_runtime.strip().lower() == "langgraph"
+
+
 def _hydrate_required_application_gate(
     db: Session,
     payload: StewardPlanRequest,
-    planner: StewardPlannerService,
+    planner: StewardPlannerLike,
 ) -> StewardPlanRequest:
     context_json = dict(payload.context_json or {})
     required_gate = context_json.get("required_application_gate")