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/chat/types.ts

@@ -13,6 +13,14 @@ export interface Chat {
   is_saved?: boolean;
   unread_count?: number;
   pinned_message_id?: number | null;
+  members_count?: number | null;
+  online_count?: number | null;
+  subscribers_count?: number | null;
+  counterpart_user_id?: number | null;
+  counterpart_name?: string | null;
+  counterpart_username?: string | null;
+  counterpart_is_online?: boolean | null;
+  counterpart_last_seen_at?: string | null;
   created_at: string;
 }
 
@@ -20,6 +28,19 @@ export interface DiscoverChat extends Chat {
   is_member: boolean;
 }
 
+export type ChatMemberRole = "owner" | "admin" | "member";
+
+export interface ChatMember {
+  id: number;
+  user_id: number;
+  role: ChatMemberRole;
+  joined_at: string;
+}
+
+export interface ChatDetail extends Chat {
+  members: ChatMember[];
+}
+
 export interface Message {
   id: number;
   chat_id: number;