Setting shop credits price incorrectly updates marketplace MANA price

## Description

When a creator changes the shop price of an item to a value in credits, it incorrectly updates the corresponding Marketplace listing price in MANA. The credits-to-MANA conversion appears to be applying the wrong ratio.

**Example:** Setting a shop price to **15 credits** results in the Marketplace listing being updated to **1.5 MANA**, suggesting a 10:1 conversion is being applied when it should not be (or the rate is incorrect).

## Steps to Reproduce

1. Open the shop and navigate to an item's pricing settings.
2. Change the shop credits price (e.g., set to 15 credits).
3. Observe the Marketplace MANA price — it changes to an incorrect value (e.g., 1.5 MANA).

## Expected Behavior

Changing the shop credits price should **not** affect the Marketplace MANA listing price, or if a conversion is intentional, the rate should be correct and clearly documented.

## Actual Behavior

The Marketplace MANA price is updated using what appears to be an incorrect credits-to-MANA conversion ratio (observed: 10:1, i.e., 15 credits → 1.5 MANA).

## Platform

Windows

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Reported by Roustan (via Flora Gomes)
Requested by Flora Gomes (<@U0AN6DV40FQ>)