feat: 添加 App 检查更新功能

- 前端：设置页面添加检查更新菜单项，从 pubspec.yaml 动态获取版本号
- 后端：新增 /api/v1/app/check-updates 接口，自动扫描 releases 目录对比版本
- 配置：新增 AppVersionSettings，支持通过环境变量配置版本和下载链接
- Docker：添加 releases 目录挂载

backend/src/core/config/settings.py

@@ -198,6 +198,25 @@ class DatabaseSettings(BaseModel):
         )
 
 
+class AppVersionSettings(BaseModel):
+    releases_dir: str = Field(
+        default="releases",
+        description="安装包目录，相对于项目根目录",
+    )
+    current_version: str = Field(
+        default="0.1.0",
+        description="当前版本号",
+    )
+    current_build: int = Field(
+        default=1,
+        description="当前构建号",
+    )
+    download_base_url: str = Field(
+        default="",
+        description="下载链接基础域名，如 https://your-domain.com",
+    )
+
+
 def _resolve_env_file() -> str:
     current = Path(__file__).resolve()
     for parent in [current, *current.parents]:
@@ -221,6 +240,7 @@ class Settings(BaseSettings):
     agent_runtime: AgentRuntimeSettings = AgentRuntimeSettings()
     taskiq: TaskiqSettings = TaskiqSettings()
     database: DatabaseSettings = DatabaseSettings()
+    app_version: AppVersionSettings = AppVersionSettings()
 
     @computed_field
     @property