feat(market): host and publish the cover photo with each offer

Wire the Blossom MediaTransport into publishing: shareable lots carry
their varietyId, publishLots uploads the lot's cover photo and puts the
returned URL on the offer (best-effort — a hosting failure still publishes
the offer, just without a photo). Add a configurable media server (Comunes
default, empty to opt out) to settings and the market advanced config.

apps/app_seeds/test/state/offers_cubit_test.dart

@@ -1,4 +1,5 @@
 import 'dart:async';
+import 'dart:typed_data';
 
 import 'package:commons_core/commons_core.dart';
 import 'package:flutter_test/flutter_test.dart';
@@ -40,6 +41,24 @@ class FakeOfferTransport implements OfferTransport {
   Future<void> close() async {}
 }
 
+/// In-memory [MediaTransport]: hands back a fixed URL, or throws when [fail] is
+/// set (to prove the publish step degrades to no-image instead of erroring).
+class FakeMediaTransport implements MediaTransport {
+  FakeMediaTransport({this.fail = false});
+  final bool fail;
+  int uploads = 0;
+
+  @override
+  Future<Uri> upload(Uint8List bytes, {required String mimeType}) async {
+    uploads++;
+    if (fail) throw const MediaUploadException('boom', 500);
+    return Uri.parse('https://media.test/hosted.jpg');
+  }
+
+  @override
+  Future<void> close() async {}
+}
+
 /// Emits every published offer TWICE on discover — the way a relay legitimately
 /// resends an addressable event (a stored copy plus a live echo). Used to prove
 /// the cubit dedups instead of doubling the listing.
@@ -269,12 +288,14 @@ void main() {
         const [
           ShareableLot(
             lotId: 'lot-1',
+            varietyId: 'var-1',
             summary: 'Tomate',
             offerStatus: OfferStatus.shared,
             category: 'Solanaceae',
           ),
           ShareableLot(
             lotId: 'lot-2',
+            varietyId: 'var-2',
             summary: 'Judía',
             offerStatus: OfferStatus.exchange,
           ),
@@ -291,6 +312,85 @@ void main() {
       await cubit.close();
     });
 
+    test('publishLots hosts the cover photo and puts its URL on the offer',
+        () async {
+      final transport = FakeOfferTransport();
+      final media = FakeMediaTransport();
+      final cubit = OffersCubit(
+        transport,
+        media: media,
+        coverPhoto: (id) async => Uint8List.fromList([1, 2, 3]),
+      );
+      await cubit.publishLots(
+        const [
+          ShareableLot(
+            lotId: 'lot-1',
+            varietyId: 'var-1',
+            summary: 'Tomate',
+            offerStatus: OfferStatus.shared,
+          ),
+        ],
+        authorPubkeyHex: 'ab' * 32,
+        areaGeohash: 'sp3e9',
+      );
+
+      await cubit.discover('sp3');
+      await pumpEventQueue();
+      expect(media.uploads, 1);
+      expect(cubit.state.offers.single.imageUrl, 'https://media.test/hosted.jpg');
+      await cubit.close();
+    });
+
+    test('publishLots still publishes (without image) when hosting fails',
+        () async {
+      final transport = FakeOfferTransport();
+      final cubit = OffersCubit(
+        transport,
+        media: FakeMediaTransport(fail: true),
+        coverPhoto: (id) async => Uint8List.fromList([1, 2, 3]),
+      );
+      final count = await cubit.publishLots(
+        const [
+          ShareableLot(
+            lotId: 'lot-1',
+            varietyId: 'var-1',
+            summary: 'Tomate',
+            offerStatus: OfferStatus.shared,
+          ),
+        ],
+        authorPubkeyHex: 'ab' * 32,
+        areaGeohash: 'sp3e9',
+      );
+
+      expect(count, 1); // the offer still went out
+      await cubit.discover('sp3');
+      await pumpEventQueue();
+      expect(cubit.state.offers.single.imageUrl, isNull);
+      await cubit.close();
+    });
+
+    test('publishLots without a media server publishes no image', () async {
+      final transport = FakeOfferTransport();
+      final cubit = OffersCubit(transport); // no media, no coverPhoto
+      await cubit.publishLots(
+        const [
+          ShareableLot(
+            lotId: 'lot-1',
+            varietyId: 'var-1',
+            summary: 'Tomate',
+            offerStatus: OfferStatus.shared,
+          ),
+        ],
+        authorPubkeyHex: 'ab' * 32,
+        areaGeohash: 'sp3e9',
+      );
+
+      await cubit.discover('sp3');
+      await pumpEventQueue();
+      expect(cubit.state.offers.single.imageUrl, isNull);
+      await cubit.close();
+    });
+
     test('publishLots is a no-op offline or with no area', () async {
       final offline = OffersCubit(null);
       expect(
@@ -298,6 +398,7 @@ void main() {
           const [
             ShareableLot(
               lotId: 'x',
+              varietyId: 'vx',
               summary: 'x',
               offerStatus: OfferStatus.shared,
             ),
@@ -315,6 +416,7 @@ void main() {
           const [
             ShareableLot(
               lotId: 'x',
+              varietyId: 'vx',
               summary: 'x',
               offerStatus: OfferStatus.shared,
             ),
@@ -336,13 +438,20 @@ void main() {
         cubit: cubit,
         shareableLots: const [
           ShareableLot(
-              lotId: 'lot-1', summary: 'Tomate', offerStatus: OfferStatus.shared),
+              lotId: 'lot-1',
+              varietyId: 'var-1',
+              summary: 'Tomate',
+              offerStatus: OfferStatus.shared),
           ShareableLot(
               lotId: 'lot-2',
+              varietyId: 'var-2',
               summary: 'Judía',
               offerStatus: OfferStatus.exchange),
           ShareableLot(
-              lotId: 'lot-3', summary: 'Otro', offerStatus: OfferStatus.shared),
+              lotId: 'lot-3',
+              varietyId: 'var-3',
+              summary: 'Otro',
+              offerStatus: OfferStatus.shared),
         ],
         authorPubkeyHex: 'ab' * 32,
         areaGeohash: 'sp3e9',