## 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)
