feat: add real media albums across backend, web, and android

- add backend media-group endpoint with atomic message and attachment creation

- switch web composer to send multi-media albums through the new endpoint

- switch Android batch media sending to upload first and create one album message with caption

web/src/api/chats.ts

@@ -106,6 +106,42 @@ export async function sendMessageWithClientId(
   return data;
 }
 
+export interface MediaGroupAttachmentInput {
+  file_url: string;
+  file_type: string;
+  file_size: number;
+  waveform_points?: number[] | null;
+}
+
+export interface MediaGroupSendResponse {
+  message: Message;
+  attachments: Array<{
+    id: number;
+    message_id: number;
+    file_url: string;
+    file_type: string;
+    file_size: number;
+    waveform_points?: number[] | null;
+  }>;
+}
+
+export async function sendMediaGroupWithClientId(
+  chatId: number,
+  attachments: MediaGroupAttachmentInput[],
+  text: string | null,
+  clientMessageId: string,
+  replyToMessageId?: number
+): Promise<MediaGroupSendResponse> {
+  const { data } = await http.post<MediaGroupSendResponse>("/messages/media-group", {
+    chat_id: chatId,
+    attachments,
+    text,
+    client_message_id: clientMessageId,
+    reply_to_message_id: replyToMessageId
+  });
+  return data;
+}
+
 export interface UploadUrlResponse {
   upload_url: string;
   file_url: string;