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/presence.py

@@ -32,3 +32,18 @@ async def is_user_online(user_id: int) -> bool:
         return bool(value and str(value).isdigit() and int(value) > 0)
     except RedisError:
         return False
+
+
+async def get_users_online_map(user_ids: list[int]) -> dict[int, bool]:
+    if not user_ids:
+        return {}
+    try:
+        redis = get_redis_client()
+        keys = [f"presence:user:{user_id}" for user_id in user_ids]
+        values = await redis.mget(keys)
+        return {
+            user_id: bool(value and str(value).isdigit() and int(value) > 0)
+            for user_id, value in zip(user_ids, values, strict=False)
+        }
+    except RedisError:
+        return {user_id: False for user_id in user_ids}