fix(channels): eliminate N+1 query in get_pinned_messages endpoint (#20459)

Replaced per-message user lookup with batch fetch using SQL IN clause.

Changes:
- Fetch all message user_ids in a single pass
- Use Users.get_users_by_user_ids() for batch lookup
- Build user mapping to avoid DB calls in loop
- Add early return for empty message lists

Performance: Reduces N+1 queries to 2 queries (messages + users)

backend/open_webui/routers/channels.py

@@ -843,14 +843,16 @@ async def get_pinned_channel_messages(
     limit = PAGE_ITEM_COUNT_PINNED
 
     message_list = Messages.get_pinned_messages_by_channel_id(id, skip, limit, db=db)
-    users = {}
+
+    if not message_list:
+        return []
+
+    # Batch fetch all users in a single query (fixes N+1 problem)
+    user_ids = list(set(m.user_id for m in message_list))
+    users = {u.id: u for u in Users.get_users_by_user_ids(user_ids, db=db)}
 
     messages = []
     for message in message_list:
-        if message.user_id not in users:
-            user = Users.get_user_by_id(message.user_id, db=db)
-            users[message.user_id] = user
-
         messages.append(
             MessageWithReactionsResponse(
                 **{