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_registry.py

@@ -0,0 +1,110 @@
+from __future__ import annotations
+
+from typing import Any, Callable
+
+from app.services.scenes.gate_rules import GateRule, SceneRoute
+from app.services.scenes.scene_descriptor import SceneDescriptor
+
+
+class SceneRegistry:
+    """场景注册表单例。
+
+    所有场景在 import 时注册,门控/路由/执行/字段过滤全部从这里查询。
+    gate_classify 节点是它的唯一消费者(单一决策点)。
+    """
+
+    def __init__(self) -> None:
+        self._scenes: dict[str, SceneDescriptor] = {}
+        self._flow_to_scene: dict[str, str] = {}
+
+    # ---- 注册 ----
+
+    def register(self, descriptor: SceneDescriptor) -> SceneDescriptor:
+        self._scenes[descriptor.scene_id] = descriptor
+        if descriptor.flow_id:
+            self._flow_to_scene[descriptor.flow_id] = descriptor.scene_id
+        return descriptor
+
+    # ---- 查询 ----
+
+    def get(self, scene_id: str) -> SceneDescriptor | None:
+        return self._scenes.get(str(scene_id or "").strip())
+
+    def all_scenes(self) -> list[SceneDescriptor]:
+        return list(self._scenes.values())
+
+    def scenes_sorted_by_priority(self) -> list[SceneDescriptor]:
+        """按 priority 升序排列(数字小优先)。"""
+        return sorted(self._scenes.values(), key=lambda s: s.priority)
+
+    def all_scene_ids(self) -> list[str]:
+        return [s.scene_id for s in self._scenes.values()]
+
+    def all_assigned_agents(self) -> list[str]:
+        return [s.assigned_agent for s in self._scenes.values() if s.assigned_agent]
+
+    def all_flow_ids(self) -> list[str]:
+        return [s.flow_id for s in self._scenes.values() if s.flow_id]
+
+    def all_signal_keywords(self) -> set[str]:
+        keywords: set[str] = set()
+        for scene in self._scenes.values():
+            keywords.update(scene.signal_keywords)
+        return keywords
+
+    def all_side_effect_actions(self) -> set[str]:
+        actions: set[str] = set()
+        for scene in self._scenes.values():
+            actions.update(scene.side_effect_actions)
+        return actions
+
+    def all_noop_actions(self) -> set[str]:
+        actions: set[str] = set()
+        for scene in self._scenes.values():
+            actions.update(scene.noop_actions)
+        return actions
+
+    def resolve_scene_by_action(self, action_type: str) -> SceneDescriptor | None:
+        normalized = str(action_type or "").strip()
+        for scene in self._scenes.values():
+            if normalized in scene.side_effect_actions or normalized in scene.noop_actions:
+                return scene
+        return None
+
+    def resolve_scene_by_flow(self, flow_id: str) -> SceneDescriptor | None:
+        scene_id = self._flow_to_scene.get(str(flow_id or "").strip())
+        return self.get(scene_id) if scene_id else None
+
+    def field_allowlist_for(
+        self,
+        scene_id: str,
+        *,
+        fallback: frozenset[str] | None = None,
+    ) -> frozenset[str]:
+        scene = self.get(scene_id)
+        if scene and scene.ontology_fields:
+            return frozenset(scene.ontology_fields)
+        return fallback or frozenset()
+
+    def resumable_scenes(self) -> list[SceneDescriptor]:
+        """返回所有声明了 can_resume=True 的场景。"""
+        return [s for s in self._scenes.values() if s.can_resume]
+
+    def prompt_fragments(self) -> str:
+        """拼接所有场景的 prompt_fragment,供 system prompt 注入。"""
+        fragments = [s.prompt_fragment for s in self._scenes.values() if s.prompt_fragment]
+        return "".join(fragments)
+
+    def intent_summary(self) -> str:
+        """拼接场景列表摘要,供 system prompt 引用。"""
+        fragments = [f"{s.scene_id}（{s.label}）" for s in self._scenes.values()]
+        return "、".join(fragments) if fragments else "（暂无已注册场景）"
+
+
+# 全局单例
+REGISTRY = SceneRegistry()
+
+
+def register_scene(descriptor: SceneDescriptor) -> SceneDescriptor:
+    """注册场景到全局单例。"""
+    return REGISTRY.register(descriptor)