feat: 装配切换——main/服务构造线/conftest 迁至 Registry（含默认租户过渡兜底）

- main.py: 删全局 engine/async_session;lifespan 装配 TenantRegistry
  (seed_profiles → 仓库根 tenants/,路径锚定),预热默认租户 jiancai 挂
  app.state.default_runtime,逐 active 租户(库文件已存在)跑
  sweep_stale_pending,退出 dispose_all。app.state.registry 预置即沿用
  (测试注入缝)
- routes/runtime.py(过渡,Task 7 删): get_runtime——state.tenant 优先,
  缺省落 default_runtime;chat.py/chat_rest.py 全部改走它,消除
  main↔routes 循环依赖
- ConversationManager 构造线: 新增 runtime 可选参(持 session_factory/
  compiled_wall/config 引用;prompts 仍模块级,Task 6;业务接线 Task 9),
  fire_pending_pushes 随任务传 session_factory(后台写回派发租户的库)
- lead_push_service: 去 app.main import;写库入口显式收 session_factory,
  模块级 async_session=None 仅作测试注入兜底
- conftest: client 夹具 = 每测试独立 Registry(tmp data_dir)+ 真实
  lifespan 在 TestClient 循环内预热;删 _dispose_main_engine;
  async_db 保持 NullPool(跨 asyncio.run 循环安全,刻意不换 Registry 引擎)
- 测试仅动夹具/桩签名(断言零改动): wiring 桩补 session_factory 形参;
  history-404 用例改经真实 WS 路由造 widget 会话(引擎常驻连接绑定
  TestClient 循环,测试线程不得直开 session_factory)

Co-Authored-By: Claude Code <noreply@anthropic.com>

app/engine/services/conversation.py

@@ -3,6 +3,7 @@ import re
 import uuid
 from collections.abc import AsyncGenerator
 from datetime import datetime, timedelta, timezone
+from typing import TYPE_CHECKING
 
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -26,6 +27,9 @@ from app.engine.services.contact_pipeline import (
     normalize_contacts, resolve_by_recency, scan_user_messages, split_validate,
 )
 
+if TYPE_CHECKING:
+    from app.platform.registry import TenantRuntime
+
 logger = logging.getLogger(__name__)
 
 # 职能边界触发词(Task 13.2, 评审#1/#4 修订): 命中即注入红线提醒。精修说明:
@@ -323,9 +327,20 @@ def _boundary_rotate() -> int:
 
 
 class ConversationManager:
-    def __init__(self, db: AsyncSession, ai_service: AIService | None = None):
+    def __init__(self, db: AsyncSession, ai_service: AIService | None = None,
+                 runtime: "TenantRuntime | None" = None):
         self.db = db
         self.ai = ai_service or AIService()
+        # Task 5b 装配线注入: 持有租户运行时引用(session_factory/compiled_wall/
+        # config 就位;prompts 仍为模块级,Task 6 才 PromptProvider 化;email/push
+        # 等 settings→config 业务接线是 Task 9)。None = 无租户上下文(直连测试)。
+        self.runtime = runtime
+
+    @property
+    def session_factory(self):
+        """推送后台任务写结果用的工厂(写回派发租户的库);无 runtime 时
+        None,lead_push_service 回退模块级兜底。"""
+        return self.runtime.session_factory if self.runtime is not None else None
 
     async def create_conversation(
         self,
@@ -777,8 +792,9 @@ class ConversationManager:
         await self.db.commit()
 
         # 询盘推送(设计 8.1): 主事务提交后才派生后台任务——payload 内存
-        # 传递,任务不读库只写结果;fire 为同步函数,纳秒级返回不阻塞
-        lead_push_service.fire_pending_pushes(pending_pushes)
+        # 传递,任务不读库只写结果;fire 为同步函数,纳秒级返回不阻塞。
+        # session_factory 随任务传递: 写回本租户的库(Task 5b)
+        lead_push_service.fire_pending_pushes(pending_pushes, self.session_factory)
 
         yield {
             "type": "evaluated",