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

app/chats/schemas.py

@@ -18,6 +18,14 @@ class ChatRead(BaseModel):
     is_saved: bool = False
     unread_count: int = 0
     pinned_message_id: int | None = None
+    members_count: int | None = None
+    online_count: int | None = None
+    subscribers_count: int | None = None
+    counterpart_user_id: int | None = None
+    counterpart_name: str | None = None
+    counterpart_username: str | None = None
+    counterpart_is_online: bool | None = None
+    counterpart_last_seen_at: datetime | None = None
     created_at: datetime