feat(offers): asking price on lots, published with sale offers (schema v11)

The data model always meant sale offers to carry an informational price
(data-model §2.8, sharing-model §4.4); with offer state collapsed onto
the Lot, the price lives there. Null amount on a sell lot = 'to be
agreed' — the NIP-99 event simply omits the price tag.

- schema v11: Lots.price_amount/price_currency, guarded migration,
  schema dump + generated schema_v11 for the round-trip test
- ShareableLot/VarietyLot carry the price (sell-only in shareableLots);
  addLot/updateLot persist it and clear it when the lot leaves sale
- OffersCubit.publishLots forwards it (outbox flush picks it up free)
- lot sheet: price + currency inputs revealed only on 'For sale', with
  the €/Ğ1/hours quick-picks extracted to a shared CurrencyQuickPicks
  widget (reused by the sale sheet)
- backup JSON round-trips the new columns
- i18n share.price/priceHint in en/es/pt/ast

apps/app_seeds/lib/i18n/ast.i18n.json

@@ -303,6 +303,8 @@
     "add": "¿Compártesla?",
     "title": "¿Compártesla?",
     "nudge": "Tienes a esgaya: podríes compartir un poco.",
+    "price": "Preciu",
+    "priceHint": "Déxalu baleru p'alcordalu depués",
     "private": "Namás pa min",
     "gift": "Pa regalar",
     "exchange": "Pa trocar",