feat(messages): stream history one-message-per-WS-frame instead of 50-row batch

- backend: add streamMany generator (paginated, yields one record at a time)
  + messagesService.streamMessages + WS 'stream_messages' handler emitting
  'message_snapshot' per message, 'message_snapshot_end' with nextCursor
- frontend: useMessagesStream hook accumulates snapshots into SWR list,
  SSR getMessages seeds first paint, WsHook gains sendText
- add stream-many.test.ts locking the one-at-a-time + cursor contract

services/frontend/src/app/(dashboard)/messages/page.tsx

@@ -1,5 +1,5 @@
 import { PageTransition } from "@/components/shared";
-import { getConfig, getGuilds } from "@/lib/api/server";
+import { getConfig, getGuilds, getMessages } from "@/lib/api/server";
 import { MessagesView } from "./view";
 
 export const dynamic = "force-dynamic";
@@ -7,8 +7,16 @@ export const dynamic = "force-dynamic";
 export default async function MessagesPage() {
   let config: import("@/lib/types/guild").AppConfig | undefined;
   let guilds: import("@/lib/types").Guild[] | undefined;
+  let initialMessages: {
+    data: import("@/lib/types").MessageRecord[];
+    nextCursor: string | null;
+  } | null = null;
   try {
     [config, guilds] = await Promise.all([getConfig(), getGuilds()]);
+    const gid = config?.monitorGuildId;
+    if (gid) {
+      initialMessages = await getMessages(gid, undefined, 50);
+    }
   } catch {
     /* client hooks surface errors */
   }
@@ -17,6 +25,7 @@ export default async function MessagesPage() {
       <MessagesView
         initialGuilds={guilds}
         initialGuildId={config?.monitorGuildId ?? null}
+        initialMessages={initialMessages}
       />
     </PageTransition>
   );