fix: Adding a price silently fails

[chetanr25]

What

• Adding a price doesn't work the price is queued but never actually uploaded.
• Root cause: in BackgroundTaskPrice.fromJson, json[_jsonTagBarcodes] is cast as List<String>, this should be List<dynamic>

Steps to reproduce the behavior

1. Go to a product page
2. Tap "Add a price"
3. Fill in the price details and submit
4. Price appears queued but is never uploaded

Expected behavior

The price should be successfully uploaded to Open Prices.

Additional context

Fix: change as List<String> to as List<dynamic> in BackgroundTaskPrice.fromJson (only .length is needed from the list). This bug was introduced in PR #7408