Change UpdateUserMaxDevices to return updated user record

- Change query from :exec to :one with RETURNING *
- Allows handler to detect when user doesn't exist
- Follows pattern established by UpdateMediaItem
- Required for 404 response on non-existent user

Related: Fix for TestUpdateUserMaxDevicesNonExistentUser

internal/database/queries/queries.sql

@@ -307,8 +307,9 @@ UPDATE users SET email = $2, updated_at = NOW() WHERE id = $1;
 -- name: UpdatePassword :exec
 UPDATE users SET password_hash = $2, updated_at = NOW() WHERE id = $1;
 
--- name: UpdateUserMaxDevices :exec
+-- name: UpdateUserMaxDevices :one
-UPDATE users SET max_devices = $2, updated_at = NOW() WHERE id = $1;
+UPDATE users SET max_devices = $2, updated_at = NOW() WHERE id = $1
+RETURNING *;
 
 -- name: CountUserDevices :one
 SELECT COUNT(*) FROM devices WHERE user_id = $1;