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/repository.py

@@ -105,6 +105,11 @@ async def list_chat_members(db: AsyncSession, *, chat_id: int) -> list[ChatMembe
     return list(result.scalars().all())
 
 
+async def list_chat_member_user_ids(db: AsyncSession, *, chat_id: int) -> list[int]:
+    result = await db.execute(select(ChatMember.user_id).where(ChatMember.chat_id == chat_id))
+    return list(result.scalars().all())
+
+
 async def list_user_chat_ids(db: AsyncSession, *, user_id: int) -> list[int]:
     result = await db.execute(
         select(ChatMember.chat_id).where(ChatMember.user_id == user_id).order_by(ChatMember.chat_id.asc())