implemented backward pagination for chat history with on-scroll prefetching

lib/api/marianumcloud/talk/chat/get_chat_cache.dart

@@ -16,7 +16,10 @@ class GetChatCache extends SimpleCache<GetChatResponse> {
            GetChatParams(
              lookIntoFuture: GetChatParamsSwitch.off,
              setReadMarker: GetChatParamsSwitch.on,
-             limit: 200,
+             // Small initial page; also the per-chat offline snapshot written to
+             // localstore. Older messages are paged in on scroll-up via
+             // GetChatHistory. Keep in sync with ChatBloc's _kInitialPageSize.
+             limit: 50,
            ),
          ).run(),
          fromJson: GetChatResponse.fromJson,

lib/api/marianumcloud/talk/chat/get_chat_history.dart

@@ -0,0 +1,56 @@
+import 'package:http/http.dart' as http;
+
+import '../../../errors/server_exception.dart';
+import '../../../http_errors.dart';
+import '../../nextcloud_ocs.dart';
+import 'get_chat_params.dart';
+import 'get_chat_response.dart';
+
+/// Backwards-paging variant of GetChat (`lookIntoFuture=0` + `lastKnownMessageId`)
+/// that fetches the page of messages *older* than a given id. Bypasses [TalkApi]
+/// because that layer treats non-2xx as errors, and the server answers HTTP 304
+/// when there are no older messages left — a normal "start of chat" outcome here.
+/// `setReadMarker=off` so paging into history never moves the read cursor.
+class GetChatHistory {
+  final String chatToken;
+  final int lastKnownMessageId;
+  final int limit;
+
+  GetChatHistory({
+    required this.chatToken,
+    required this.lastKnownMessageId,
+    required this.limit,
+  });
+
+  /// Returns the older page, or `null` on HTTP 304 (no older messages).
+  Future<GetChatResponse?> run() async {
+    final params = GetChatParams(
+      lookIntoFuture: GetChatParamsSwitch.off,
+      lastKnownMessageId: lastKnownMessageId,
+      includeLastKnown: GetChatParamsSwitch.off,
+      setReadMarker: GetChatParamsSwitch.off,
+      limit: limit,
+    );
+    final uri = NextcloudOcs.uri(
+      'apps/spreed/api/v1/chat/$chatToken',
+      queryParameters: params.toJson(),
+    );
+    final headers = NextcloudOcs.headers();
+
+    final response = (await sendGuarded(
+      'GetChatHistory $uri',
+      () => http.get(uri, headers: headers),
+    ))!;
+
+    final status = response.statusCode;
+    if (status == 304) return null;
+    if (status >= 200 && status < 300) {
+      return GetChatResponse.fromJson(NextcloudOcs.decode(response.body))
+        ..headers = response.headers;
+    }
+    throw ServerException(
+      statusCode: status,
+      technicalDetails: 'GetChatHistory $uri: HTTP $status',
+    );
+  }
+}