fix(config): 运行时读取 SessionLocal 修复后台配置接口500 #2246 (#2248)

## 背景
#2247 合并部署到 staging 后，`GET /api/v1/admin/ditto-emotion/config` 返回 500。

## 根因
`system_config_service.py` 在**模块导入时**执行 `from ...session import SessionLocal`，此刻 `SessionLocal` 还是 `None`；之后 `initialize_database()` 才给它赋值。import 时绑定把旧的 `None` 固化，服务 fallback 取到的始终是 None，抛出 `RuntimeError(数据库会话工厂未初始化)`。

## 修复
- 改为 `from packages.adapters.sqlalchemy_impl import session as db_session`，运行时读取 `db_session.SessionLocal`。
- 补 2 个回归用例：模拟「导入时 None、启动后才赋值」的真实启动顺序；无工厂时抛 RuntimeError。

## 测试
- `tests/unit/test_system_config.py`：**29 passed**（原 27 + 新增 2）。
- black/isort/ruff 通过。

关联 #2246。

packages/application/system_config_service.py

@@ -12,7 +12,7 @@ import logging
 import threading
 from typing import Any
 
-from packages.adapters.sqlalchemy_impl.session import SessionLocal
+from packages.adapters.sqlalchemy_impl import session as db_session
 from packages.adapters.sqlalchemy_impl.system_setting_repository import (
     SQLAlchemySystemSettingRepository,
 )
@@ -35,7 +35,9 @@ class SystemConfigService:
 
     # ── 会话 ─────────────────────────────────────────────────────
     def _get_session_factory(self):
-        factory = self._session_factory or SessionLocal
+        # 必须运行时读取模块属性：模块导入时 SessionLocal 还是 None，
+        # initialize_database() 之后才被赋值，import 时绑定会拿到旧值。
+        factory = self._session_factory or db_session.SessionLocal
         if factory is None:
             raise RuntimeError("数据库会话工厂未初始化")
         return factory

tests/unit/test_system_config.py

@@ -91,6 +91,37 @@ class TestConfigService:
     def test_default_when_missing(self, config_service):
         assert config_service.get_config("not_exist", "fallback") == "fallback"
 
+    def test_uses_module_session_factory_assigned_after_import(self):
+        # 回归：模块导入时 session.SessionLocal 为 None，initialize_database()
+        # 之后才赋值；服务必须运行时读取模块属性，而不是 import 时绑定旧值。
+        from packages.adapters.sqlalchemy_impl import session as db_session
+
+        engine = create_engine("sqlite://")
+        Base.metadata.create_all(engine)
+        factory = sessionmaker(bind=engine)
+        writer = SystemConfigService(session_factory=factory)
+        writer.set_config("late_key", 7, setting_type=SETTING_TYPE_INT)
+
+        reader = SystemConfigService()  # 不注入工厂，依赖模块级 SessionLocal
+        old = db_session.SessionLocal
+        try:
+            db_session.SessionLocal = factory
+            assert reader.get_config("late_key", 0) == 7
+        finally:
+            db_session.SessionLocal = old
+
+    def test_raises_when_no_session_factory(self):
+        from packages.adapters.sqlalchemy_impl import session as db_session
+
+        svc = SystemConfigService()
+        old = db_session.SessionLocal
+        try:
+            db_session.SessionLocal = None
+            with pytest.raises(RuntimeError):
+                svc.get_config("anything", 1)
+        finally:
+            db_session.SessionLocal = old
+
     def test_db_overrides_default(self, config_service):
         config_service.set_config("k", 20, setting_type=SETTING_TYPE_INT)
         # 再次读取应命中 DB 值，而非传入的默认