merge: T1 四档分级降级熔断（T-P1-25b，NFR-COST-02）

apps/api/routers/agent.py

@@ -18,6 +18,7 @@ from packages.llm.errors import (
     AllProvidersFailedError,
     CachePrefixViolationError,
     DailyBudgetExceededError,
+    SceneDegradedError,
     StructuredOutputError,
 )
 from packages.shared.errors import AppError, ErrorCode
@@ -37,6 +38,9 @@ async def chat(
         result = await agent.chat(user_id=user_id, message=payload.message, request_key=request_key)
     except DailyBudgetExceededError as exc:
         raise AppError(ErrorCode.LLM_DAILY_BUDGET_EXCEEDED) from exc
+    except SceneDegradedError as exc:
+        # 分级降级（§13.3 tier2/3）：该场景今日被关停，但平台未爆预算
+        raise AppError(ErrorCode.LLM_SCENE_DEGRADED) from exc
     except StructuredOutputError as exc:
         raise AppError(ErrorCode.LLM_STRUCTURED_OUTPUT_FAILED) from exc
     except AllProvidersFailedError as exc: