Add HealthChat API and extended profile services (#9)

Expose OpenID tokens and homeserver credentials from the client proxy, and
add fork-only services for the HealthChat member directory backend
(api.healthchat.ch) plus a service that reads MSC4133 extended profile
fields directly from the homeserver. Groundwork for the members tab and
the profile screens.

Claude-Session: https://claude.ai/code/session_013qdTbAWsZj2quQzEkagXaS

ElementX/Sources/Services/Client/OpenIDToken.swift

@@ -0,0 +1,36 @@
+//
+// Copyright 2026 Element Creations Ltd.
+//
+// SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+// Please see LICENSE files in the repository root for full details.
+//
+
+import Foundation
+import MatrixRustSDK
+
+/// An OpenID token that third-party services can use to verify the user's identity
+/// against the homeserver (MSC1961). Short-lived — cache per call site.
+nonisolated struct OpenIDToken: Equatable {
+    let accessToken: String
+    let matrixServerName: String
+    /// The interval after which the token expires, relative to when it was requested.
+    let expiresIn: TimeInterval
+    
+    init(accessToken: String, matrixServerName: String, expiresIn: TimeInterval) {
+        self.accessToken = accessToken
+        self.matrixServerName = matrixServerName
+        self.expiresIn = expiresIn
+    }
+    
+    init(rustValue: MatrixRustSDK.OpenIdToken) {
+        accessToken = rustValue.accessToken
+        matrixServerName = rustValue.matrixServerName
+        expiresIn = TimeInterval(rustValue.expiresInSeconds)
+    }
+}
+
+extension OpenIDToken: CustomStringConvertible {
+    var description: String {
+        "OpenIDToken(matrixServerName: \(matrixServerName), accessToken: <redacted>)"
+    }
+}