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/users/repository.py

@@ -1,3 +1,5 @@
+from datetime import datetime, timezone
+
 from sqlalchemy import func, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -47,3 +49,12 @@ async def search_users_by_username(
     stmt = stmt.order_by(User.username.asc()).limit(limit)
     result = await db.execute(stmt)
     return list(result.scalars().all())
+
+
+async def update_user_last_seen_now(db: AsyncSession, *, user_id: int) -> User | None:
+    user = await get_user_by_id(db, user_id)
+    if not user:
+        return None
+    user.last_seen_at = datetime.now(timezone.utc)
+    await db.flush()
+    return user