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/db/database.dart

@@ -31,7 +31,7 @@ class AppDatabase extends _$AppDatabase {
 
   /// Current schema version; also stamped into interchange exports so an
   /// importer knows which app generation wrote the file (data-model §7).
-  static const int currentSchemaVersion = 10;
+  static const int currentSchemaVersion = 11;
 
   @override
   int get schemaVersion => currentSchemaVersion;
@@ -153,6 +153,16 @@ class AppDatabase extends _$AppDatabase {
           await m.createTable(sales);
         }
       }
+      // v11: asking price on Lots — published with "sell" market offers.
+      // Guarded (see the v7 note above).
+      if (from < 11) {
+        if (!await _hasColumn('lots', 'price_amount')) {
+          await m.addColumn(lots, lots.priceAmount);
+        }
+        if (!await _hasColumn('lots', 'price_currency')) {
+          await m.addColumn(lots, lots.priceCurrency);
+        }
+      }
     },
   );