fix(web): extension price verdict and cheaper-elsewhere skip sold-out offers #1177

Follow-up from #1170 / PR #1176 review.

Problem

apps/web/src/server/rate/extension.ts (around marketOffers = plausibleOffers(offers) at L634 and L1311) does not drop offers whose availability is out_of_stock or discontinued. So the extension's "cheaper elsewhere" list (topCheaperOffers) and the market reference in evaluateExtensionPriceVerdict can still cite a sold-out listing's last price, e.g. Buds3 Pro's FPT Shop listing at 2,694,000. The inspect route shares the same gap.

Proposed fix

Filter isOfferUnavailable (from apps/web/src/lib/product-market-stats.ts) before plausibleOffers in those paths, as #1176 does for the deal radar and best offer.

Acceptance

• Neither the extension verdict nor its cheaper-elsewhere list cites an out_of_stock or discontinued offer.
• A test pins it.