feat!: genesis-pinned operator set, server identity with an encrypted control link, key-less join codes; release key continuity

Join codes need no passkey touch and carry only public values (genesis
fingerprint, owner credentials, installer hashes). Nodes pin the genesis and
owners at enrollment and only follow passkey-signed operator set versions.
The panel's server key is listed in the set; pinned nodes require its
signature and run the control link under AES-256-GCM. Password sessions carry
no capabilities. The updater adopts a next_key from a signed manifest.

Bump version to 0.2.0. [CI SKIP]

backend/fleet/src/Fleet.Identity/NodeIdentity.cs

@@ -7,6 +7,9 @@ namespace Fleet.Identity;
 public static class SignaturePurpose
 {
     public const string Auth = "auth";
+
+    /// <summary>安全链路的认证：签名同时覆盖服务器和节点的临时公钥，把这次认证和这条加密链路绑在一起。</summary>
+    public const string AuthSecure = "auth2";
     public const string Peer = "peer";
     public const string Enroll = "enroll";
 }