fix: resolve N+1 query pattern in users endpoint (#20427)

## Summary

Fixed N+1 query pattern in the `/api/v1/users` endpoint where groups were being fetched for each user individually.

### Problem

The `GET /api/v1/users` endpoint called `Groups.get_groups_by_member_id()` for each user, resulting in:
- 1 query for users
- N queries for groups (one per user)

### Solution

Added a new `Groups.get_groups_by_member_ids()` method that fetches groups for multiple users in a single query using SQL `IN` clause and `JOIN`.

### Changes

- **[groups.py](open_webui/models/groups.py)**: Added `get_groups_by_member_ids()` method
- **[users.py](open_webui/routers/users.py)**: Updated endpoint to use bulk method

### Result

- Before: 1 + N queries
- After: 2 queries total (1 for users, 1 for all groups)

backend/open_webui/routers/users.py

@@ -84,15 +84,16 @@ async def get_users(
     users = result["users"]
     total = result["total"]
 
+    # Fetch groups for all users in a single query to avoid N+1
+    user_ids = [user.id for user in users]
+    user_groups = Groups.get_groups_by_member_ids(user_ids, db=db)
+
     return {
         "users": [
             UserGroupIdsModel(
                 **{
                     **user.model_dump(),
-                    "group_ids": [
-                        group.id
-                        for group in Groups.get_groups_by_member_id(user.id, db=db)
-                    ],
+                    "group_ids": [group.id for group in user_groups.get(user.id, [])],
                 }
             )
             for user in users