feat: install nodes and the backend from a static compose file; remove dead code [CI]

Agent, Probe and Backend now start from one static compose.yaml plus a local
.env: the panel (or the release assets, for the very first Backend) provides
the file, the panel's command writes the public values into .env, checks the
file's SHA-256 and runs compose up. Updating is compose pull && compose up -d.
The image is chosen by the CI channel (:latest, or MOSHIRO_VERSION in .env),
not by the panel; the previous one-line script stays as a fallback.

- packaging/compose/{agent,probe,backend}/compose.yaml are the single source,
  embedded in the Backend and served at /install/compose/{file}; the root
  compose.yaml moved there. Releases also carry them and list them in the
  signed SHA256SUMS.
- The container entrypoint defaults the node name to the host name.
- Join token creation returns a compose block (.env, url, hash, per-platform
  commands); the admin dialog leads with it.
- Remove code with no references (HotLayer members, LiveHub.SubscriberCount,
  QueryService.ParseParams, ReleaseService.Forget, a few constants) and reduce
  the install-code lookup to the role it is for.

backend/src/MoShiro.Backend.Api/InstallScripts.cs

@@ -19,6 +19,12 @@ public static class InstallScripts
 
     public static string Script(string resource) => Read(resource);
 
+    /// <summary>compose 文件的资源名（<c>agent.yaml</c>、<c>probe.yaml</c>、<c>backend.yaml</c> 对应的）；不是这三个就是 null。</summary>
+    public static string? ComposeResource(string file) => file is "agent.yaml" or "probe.yaml" or "backend.yaml" ? "compose-" + file : null;
+
+    /// <summary>某个角色（agent、probe）的 compose 文件的 SHA-256；其他角色返回空。</summary>
+    public static string ComposeSha256(string? role) => role is "agent" or "probe" ? Sha256("compose-" + role + ".yaml") : string.Empty;
+
     /// <summary>脚本内容的 SHA-256（小写十六进制，按 UTF-8 字节）。</summary>
     public static string Sha256(string resource) => Convert.ToHexStringLower(SHA256.HashData(Encoding.UTF8.GetBytes(Read(resource))));