Read-only eligible-offers endpoint for badge campaigns (Smart! Weeks etc.)

[yousellpolska]

Following up on #13424 - that issue is resolved (the documented /sale/badges endpoints work as expected). @PrzemyslawLukanowski suggested moving the remaining feature request here.

Context

For AlleObnizka there is GET /sale/alle-discount/{campaignId}/eligible-offers, which returns, per offer and before any submission: eligibility, requiredMerchantPrice, and the minimum guaranteed discount percentage. An integrator can therefore decide whether an offer qualifies and at what price without submitting anything.

For badge campaigns (the /sale/badges family - Smart! Weeks and similar) there is no equivalent. The seller panel UI (Reklama i promowanie ? Kampanie i programy ? campaign ? Zglos oferty) clearly has this data - it shows, per offer, before submission:

• najnizsza cena oferty z ostatnich 30 dni - the omnibus MIN30d base
• cena promocyjna - the suggested promotional price
• warunki uczestnictwa - eligibility flag (meets / does not meet)
• limit na kupujacego - purchase limit

None of this is reachable from the API. The only workarounds are both poor:

1. Replicate MIN30d from our own order history - fragile: it assumes complete order sync, assumes Allegro's omnibus formula will not change, and offers with no recent transactions give no signal at all.
2. Probe by submitting and parsing rejectionReasons - this pollutes submission history with noise.

Suggestion

A read-only endpoint, mirroring the AlleObnizka one:

GET /sale/badge-campaigns/{campaignId}/eligible-offers
  ? [{ offer: { id },
        eligibility: { eligible, reasons[] },
        prices: { marketPrice, suggestedBargain } }]

Why it matters

This is the missing piece for any automated repricing / dynamic-pricing integration: to set a price programmatically you first need the campaign-eligible thresholds that the panel already computes and enforces. Today badge campaigns can be operated reliably only from the UI, not from the API - the submission endpoints exist, but the data needed to submit correctly does not.

Originally raised in #13424.

• Tomasz, YOU SELL

[Lukasz-Zurek]

Thanks for your suggestion.

If we decide to implement this solution, we will inform you about it in the thread.