refactor(server): scene 注册表骨架 + 统一门控管道设计文档

Phase 1 P1.1-P1.2：为后端门控收口提供声明式场景注册基础设施。

- 新建 scenes/ 目录：gate_rules（GateRule/SceneRoute 枚举）、scene_descriptor（SceneDescriptor dataclass）、scene_registry（SceneRegistry 单例）
- 3 个场景迁入 descriptor：expense_application / reimbursement / query_travel_standard
- __init__.py 的 bootstrap_scenes 在 import 时注册 + 运行时绑定 handler/builder/executor（解决循环 import）
- 查询场景 priority=50 优先于 MODEL_ONLY 场景，确保规则匹配先于 LLM
- 落地 UNIFIED_GATE_PIPELINE.md 架构文档：目标架构 / 验收标准（接入 O(1)）/ 3 阶段迁移路径
- 76 passed，scene 注册表未破坏现有代码；与 intent_registry 暂时并存，P1.3-P1.8 会统一迁移

server/src/app/services/scenes/scene_expense_application.py

@@ -0,0 +1,38 @@
+from __future__ import annotations
+
+from app.services.scenes.gate_rules import GateRule, SceneRoute
+from app.services.scenes.scene_descriptor import SceneDescriptor
+from app.services.scenes.scene_registry import register_scene
+
+
+def register() -> None:
+    register_scene(
+        SceneDescriptor(
+            scene_id="expense_application",
+            label="费用申请",
+            assigned_agent="application_assistant",
+            signal_keywords=(
+                "申请", "出差", "差旅", "费用", "交通", "住宿", "采购", "会务", "会议",
+                "客户现场", "项目", "拜访", "调研", "驻场", "上线", "验收",
+            ),
+            ontology_fields=(),  # 沿用全局 BUSINESS_CANONICAL_FIELDS,运行时 fallback
+            gate=GateRule.MODEL_ONLY,
+            route=SceneRoute.MODEL_INTENT,
+            handler=None,
+            action_steps_builder=None,  # 运行时从 StewardActionPlanBuilder 取
+            can_resume=True,
+            flow_id="travel_application",
+            side_effect_actions=("save_application_draft", "submit_application", "run_duplicate_precheck"),
+            noop_actions=(
+                "fill_application_fields",
+                "build_application_preview",
+                "validate_required_fields",
+            ),
+            executor=None,  # 运行时从 StewardActionExecutor 取
+            prompt_fragment=(
+                "用户描述未来出差、差旅计划、去某地几天、部署、支撑、拜访或会议安排时,"
+                "即使没有出现“申请”两个字，也必须优先识别为 expense_application。"
+            ),
+            priority=100,
+        )
+    )