feat(chat): add presence metadata and improve web chat core

- add user last_seen_at with alembic migration and persist on realtime disconnect
- extend chat serialization with private online/last_seen, group members/online, channel subscribers
- add Redis batch presence lookup helper
- update web chat list/header to display status counters and last-seen labels
- improve delivery receipt handling using last_delivered/last_read boundaries
- include chat info panel and related API/type updates

web/src/api/users.ts

@@ -19,3 +19,8 @@ export async function updateMyProfile(payload: UserProfileUpdatePayload): Promis
   const { data } = await http.put<AuthUser>("/users/profile", payload);
   return data;
 }
+
+export async function getUserById(userId: number): Promise<AuthUser> {
+  const { data } = await http.get<AuthUser>(`/users/${userId}`);
+  return data;
+}