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/HealthChat/ExtendedProfileServiceProtocol.swift

@@ -0,0 +1,20 @@
+//
+// 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
+
+enum ExtendedProfileServiceError: Error, Equatable {
+    case missingCredentials
+    case networkFailure
+    case decodingFailure
+    case httpStatus(Int)
+}
+
+// sourcery: AutoMockable
+protocol ExtendedProfileServiceProtocol {
+    func profile(for userID: String) async -> Result<ExtendedProfile, ExtendedProfileServiceError>
+}