feat(backorders): BACK-466 render backorder message on PDP

[bc-apostoliuk]

Jira: BACK-466

What/Why?

Screen.Recording.2026-03-19.at.19.26.45.mov

Renders a custom backorder message alongside the existing qty-backordered message on the Product Detail Page.

• backorder_messages list and product.show_backorder_message are injected into the JS context from product-view.html
• The data-product-backordered row is split into two inner spans: data-qty-backordered-message (existing qty text) and data-backorder-message (new custom message)
• updateBackorderMessage() looks up backorder_message_id (returned by remote/v1/product-attributes/:productId) in the injected list and renders it only when the qty backordered span is also visible
• toggleBackorderedContainer() is now driven solely by the qty span, so the row only shows when units are actually being backordered
• updateBackorderContext() always overwrites backorderMessageId with data.backorder_message_id ?? null to prevent stale IDs from a previous variant bleeding through on variant switches
• updateBackorderMessage() is called in updateView() (variant changes) and both quantity change handlers (+/- button and keyup)
• For simple products (no selectable options), an explicit optionChange call is made on page init to fetch backorder_message_id

Rollout/Rollback

Standard deployment. Rollback by reverting the PR if needed.

Testing

1. Simple product with backorder message configured — load PDP, increment qty past available stock; verify the custom backorder message appears next to "N will be backordered"
2. Complex product (variants) with per-variant backorder messages — select each variant, increment qty past stock; verify the correct message for each variant is shown
3. Variant switch — with qty in backorder range, switch variants; verify the message updates to the new variant's message (or clears if the new variant has none) after the API responds
4. Qty back below threshold — decrement qty below backordered range; verify both the qty message and the custom backorder message disappear together
5. show_backorder_message = false — verify no custom message renders even when backorder_message_id is set

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