perf(channels): batch N+1 queries for reactions and thread replies (#25831)

Replace per-message database queries with batch IN-clause queries in
channel message handlers. This eliminates the N+1 query pattern that
caused ~102 queries per channel page load (50 messages × 2 queries each).

Changes:
- Add get_reactions_by_message_ids() to MessageTable: single query
  fetches all reactions for multiple messages using IN clause with
  User JOIN, returns dict[message_id, list[Reactions]]
- Add get_thread_reply_counts_by_message_ids() to MessageTable: single
  GROUP BY aggregate query returns (count, max_created_at) per parent,
  replacing full object loads just to call len()
- Refactor get_channel_messages(): 102 → 4 queries per page
- Refactor get_pinned_channel_messages(): 22 → 3 queries per page
- Refactor get_channel_thread_messages(): 53 → 4 queries per page
- Refactor send_notification(): N+1 membership check → batch set lookup

backend/open_webui/routers/channels.py

@@ -808,10 +808,14 @@ async def get_channel_messages(
     user_ids = list(set(m.user_id for m in message_list))
     fetched_users = {u.id: u for u in await Users.get_users_by_user_ids(user_ids, db=db)}
 
+    # Batch fetch reactions and reply counts in 2 queries (fixes N+1)
+    message_ids = [m.id for m in message_list]
+    all_reactions = await Messages.get_reactions_by_message_ids(message_ids, db=db)
+    all_reply_counts = await Messages.get_thread_reply_counts_by_message_ids(message_ids, db=db)
+
     messages = []
     for message in message_list:
-        thread_replies = await Messages.get_thread_replies_by_message_id(message.id, db=db)
-        latest_thread_reply_at = thread_replies[0].created_at if thread_replies else None
+        reply_count, latest_reply_at = all_reply_counts.get(message.id, (0, None))
 
         # Use message.user if present (for webhooks), otherwise look up by user_id
         user_info = message.user
@@ -822,9 +826,9 @@ async def get_channel_messages(
             MessageUserResponse(
                 **{
                     **message.model_dump(),
-                    'reply_count': len(thread_replies),
-                    'latest_reply_at': latest_thread_reply_at,
-                    'reactions': await Messages.get_reactions_by_message_id(message.id, db=db),
+                    'reply_count': reply_count,
+                    'latest_reply_at': latest_reply_at,
+                    'reactions': all_reactions.get(message.id, []),
                     'user': user_info,
                 }
             )
@@ -873,6 +877,10 @@ async def get_pinned_channel_messages(
     user_ids = list(set(m.user_id for m in message_list))
     fetched_users = {u.id: u for u in await Users.get_users_by_user_ids(user_ids, db=db)}
 
+    # Batch fetch reactions in 1 query (fixes N+1)
+    message_ids = [m.id for m in message_list]
+    all_reactions = await Messages.get_reactions_by_message_ids(message_ids, db=db)
+
     messages = []
     for message in message_list:
         # Check for webhook identity in meta
@@ -892,7 +900,7 @@ async def get_pinned_channel_messages(
             MessageWithReactionsResponse(
                 **{
                     **message.model_dump(),
-                    'reactions': await Messages.get_reactions_by_message_id(message.id, db=db),
+                    'reactions': all_reactions.get(message.id, []),
                     'user': user_info,
                 }
             )
@@ -913,8 +921,11 @@ async def send_notification(request, channel, message, active_user_ids, db=None)
 
     users = await get_channel_users_with_access(channel, 'read', db=db)
 
+    # Batch fetch channel members in 1 query (fixes N+1)
+    member_ids = {m.user_id for m in await Channels.get_members_by_channel_id(channel.id, db=db)}
+
     for u in users:
-        if (u.id not in active_user_ids) and await Channels.is_user_channel_member(channel.id, u.id, db=db):
+        if (u.id not in active_user_ids) and u.id in member_ids:
             if enable_user_webhooks and u.settings:
                 webhook_url = u.settings.ui.get('notifications', {}).get('webhook_url', None)
                 if webhook_url:
@@ -1401,6 +1412,10 @@ async def get_channel_thread_messages(
     user_ids = list(set(m.user_id for m in message_list))
     fetched_users = {u.id: u for u in await Users.get_users_by_user_ids(user_ids, db=db)}
 
+    # Batch fetch reactions in 1 query (fixes N+1)
+    message_ids = [m.id for m in message_list]
+    all_reactions = await Messages.get_reactions_by_message_ids(message_ids, db=db)
+
     messages = []
     for message in message_list:
         # Use message.user if present (for webhooks), otherwise look up by user_id
@@ -1414,7 +1429,7 @@ async def get_channel_thread_messages(
                     **message.model_dump(),
                     'reply_count': 0,
                     'latest_reply_at': None,
-                    'reactions': await Messages.get_reactions_by_message_id(message.id, db=db),
+                    'reactions': all_reactions.get(message.id, []),
                     'user': user_info,
                 }
             )