feat: print build info at startup; version placeholders filled in by CI [CI SKIP]

backend/src/MoShiro.Backend/Program.cs

@@ -1,3 +1,10 @@
 using MoShiro.Backend;
 
-await BackendHost.Build(args).RunAsync();
+WebApplication app = BackendHost.Build(args);
+// 启动时打一行构建信息：版本、tag、提交、构建时间（和 Agent、Probe 启动时打的是同一组）
+if (app.Logger.IsEnabled(LogLevel.Information))
+{
+    app.Logger.LogInformation("moshiro backend starting: version {BuildInfo}", MoShiro.Protocol.BuildInfo.Summary);
+}
+
+await app.RunAsync();