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/realtime/service.py

@@ -9,12 +9,14 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.chats.repository import list_user_chat_ids
 from app.chats.service import ensure_chat_membership
+from app.database.session import AsyncSessionLocal
 from app.messages.schemas import MessageCreateRequest, MessageRead, MessageStatusUpdateRequest
 from app.messages.service import create_chat_message, mark_message_status
 from app.realtime.models import ConnectionContext
 from app.realtime.presence import mark_user_offline, mark_user_online
 from app.realtime.repository import RedisRealtimeRepository
 from app.realtime.schemas import ChatEventPayload, MessageStatusPayload, OutgoingRealtimeEvent, SendMessagePayload
+from app.users.repository import update_user_last_seen_now
 
 
 class RealtimeGateway:
@@ -76,6 +78,7 @@ class RealtimeGateway:
                 if not subscribers:
                     self._chat_subscribers.pop(chat_id, None)
             await mark_user_offline(user_id)
+            await self._persist_last_seen(user_id)
 
     async def handle_send_message(self, db: AsyncSession, user_id: int, payload: SendMessagePayload) -> None:
         message = await create_chat_message(
@@ -197,6 +200,7 @@ class RealtimeGateway:
                 if not subscribers:
                     self._chat_subscribers.pop(chat_id, None)
             await mark_user_offline(user_id)
+            await self._persist_last_seen(user_id)
 
     @staticmethod
     def _extract_chat_id(channel: str) -> int | None:
@@ -207,5 +211,13 @@ class RealtimeGateway:
             return None
         return int(chat_id)
 
+    async def _persist_last_seen(self, user_id: int) -> None:
+        try:
+            async with AsyncSessionLocal() as db:
+                await update_user_last_seen_now(db, user_id=user_id)
+                await db.commit()
+        except Exception:
+            return
+
 
 realtime_gateway = RealtimeGateway()