Sidebar crashes when user has 20+ direct conversations

[ESH13]

Bug

find_direct_placeholder_users in Users::SidebarsController passes a negative value to .limit() when a user has direct rooms with 20 or more unique participants.

Steps to reproduce

1. Have a user with 20+ direct message conversations (20+ unique other users across their direct rooms)
2. Load any page that renders the sidebar

Expected behavior

Sidebar renders normally, with no direct placeholder users shown.

Actual behavior

PostgreSQL raises LIMIT must not be negative, crashing the sidebar controller. Since the sidebar is loaded via a Turbo Frame on every page, the user sees "Content missing" on every page, every browser, every device. The application is completely unusable for that user.

Cause

DIRECT_PLACEHOLDERS is 20. exclude_user_ids includes all unique users in the user's direct rooms plus the user themselves. When that count exceeds 20, DIRECT_PLACEHOLDERS - exclude_user_ids.count goes negative.

Fix

Clamp the limit to a minimum of zero so limit(0) returns an empty result set instead of raising. One-line change — happy to open a PR.