feat(realtime): live online/offline events and unified search

- add websocket events user_online/user_offline
- broadcast presence changes on first connect and final disconnect only
- apply live presence updates in web chat store and realtime hook
- move public discover into unified left search (users + groups/channels)
- remove separate Discover Chats dialog/menu entry

app/realtime/presence.py

@@ -3,26 +3,30 @@ from redis.exceptions import RedisError
 from app.utils.redis_client import get_redis_client
 
 
-async def mark_user_online(user_id: int) -> None:
+async def mark_user_online(user_id: int) -> bool:
     try:
         redis = get_redis_client()
         key = f"presence:user:{user_id}"
         count = await redis.incr(key)
         if count == 1:
             await redis.expire(key, 3600)
+            return True
+        return False
     except RedisError:
-        return
+        return False
 
 
-async def mark_user_offline(user_id: int) -> None:
+async def mark_user_offline(user_id: int) -> bool:
     try:
         redis = get_redis_client()
         key = f"presence:user:{user_id}"
         value = await redis.decr(key)
         if value <= 0:
             await redis.delete(key)
+            return True
+        return False
     except RedisError:
-        return
+        return False
 
 
 async def is_user_online(user_id: int) -> bool: