feat: add message reliability foundation

- implement idempotent message creation via client_message_id

- add persistent delivered/read receipts

- expose /messages/status and wire websocket receipt events

- update web client to send client ids and auto-ack delivered/read

web/src/store/chatStore.ts

@@ -1,5 +1,5 @@
 import { create } from "zustand";
-import { getChats, getMessages } from "../api/chats";
+import { getChats, getMessages, updateMessageStatus } from "../api/chats";
 import type { Chat, Message } from "../chat/types";
 
 interface ChatState {
@@ -26,12 +26,17 @@ export const useChatStore = create<ChatState>((set, get) => ({
   setActiveChatId: (chatId) => set({ activeChatId: chatId }),
   loadMessages: async (chatId) => {
     const messages = await getMessages(chatId);
+    const sorted = [...messages].reverse();
     set((state) => ({
       messagesByChat: {
         ...state.messagesByChat,
-        [chatId]: [...messages].reverse()
+        [chatId]: sorted
       }
     }));
+    const lastMessage = sorted[sorted.length - 1];
+    if (lastMessage) {
+      void updateMessageStatus(chatId, lastMessage.id, "message_read");
+    }
   },
   prependMessage: (chatId, message) => {
     const old = get().messagesByChat[chatId] ?? [];