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.Rpc/FleetMethods.cs

@@ -53,6 +53,14 @@ public sealed class ChallengeParams
 
     [JsonPropertyName("expires_at_ms")]
     public long ExpiresAtMs { get; set; }
+
+    /// <summary>服务器这次连接的临时 ECDH 公钥（SPKI，base64）；服务器有身份密钥时才有。</summary>
+    [JsonPropertyName("server_eph")]
+    public string? ServerEphemeral { get; set; }
+
+    /// <summary>服务器密钥对「随机数加临时公钥」的签名（base64）：节点据此确认对面是操作员授权的服务器。</summary>
+    [JsonPropertyName("server_sig")]
+    public string? ServerSignature { get; set; }
 }
 
 public sealed class AuthParams
@@ -63,6 +71,10 @@ public sealed class AuthParams
     /// <summary>对 <c>fleet|v1|auth|{node_id}|{nonce}</c> 的签名（base64）。</summary>
     [JsonPropertyName("signature")]
     public string Signature { get; set; } = "";
+
+    /// <summary>节点的临时 ECDH 公钥（SPKI，base64）：有它就是安全链路，签名覆盖双方的临时公钥，认证结果起所有帧都是加密的。</summary>
+    [JsonPropertyName("node_eph")]
+    public string? NodeEphemeral { get; set; }
 }
 
 public sealed class AuthResult