refactor: remove the runtime binary binding; every release registration path verifies the CI signature [CI]

The hash a node reports about itself cannot be trusted (a swapped binary can
copy the old hash), so the runtime binding only caught files that were replaced
by something that did not lie. Provenance stays where it can be verified: the
signed release manifest (digest) and the signed SHA256SUMS.

- Remove BinaryBindingService, the node_bindings table (schema v15 drops it),
  BinaryPolicy, the handshake hash/manifest fields and the admin binary column.
- Drop the per-platform "binaries" list from the release manifest, the hash
  scripts and the updater's installed.manifest copy.
- ReleaseService.PublishAsync now verifies the release signature, so the admin
  POST /releases route is no longer an unsigned path; tests sign with a test key.
- Docs: section 16 explains why the runtime check is gone.

backend/src/MoShiro.Backend/BackendHost.cs

@@ -19,7 +19,7 @@ public static class BackendHost
     public const string ConfigurationSection = "MoShiro";
     public const int MaxRequestBodyBytes = 64 * 1024;
 
-    public static WebApplication Build(string[] args, Action<BackendOptions>? configureOptions = null, TimeProvider? time = null)
+    public static WebApplication Build(string[] args, Action<BackendOptions>? configureOptions = null, TimeProvider? time = null, Action<IServiceCollection>? configureServices = null)
     {
         WebApplicationBuilder builder = WebApplication.CreateSlimBuilder(args);
 
@@ -51,6 +51,7 @@ public static class BackendHost
         services.AddMoShiroApplication(options);
         services.AddMoShiroGateway();
         services.AddMoShiroApi();
+        configureServices?.Invoke(services); // 测试替换发布公钥这类服务
 
         WebApplication app = builder.Build();
         // 静默死亡的 Agent（断电、断网）不会关闭连接：15 秒一个 Ping，20 秒没有 Pong 就关闭，这样面板上的在线状态才可信