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/lib/data/variety_repository.dart

@@ -174,6 +174,7 @@ class SeedLabelEntry extends Equatable {
 class ShareableLot extends Equatable {
   const ShareableLot({
     required this.lotId,
+    required this.varietyId,
     required this.summary,
     required this.offerStatus,
     this.category,
@@ -183,6 +184,10 @@ class ShareableLot extends Equatable {
 
   final String lotId;
 
+  /// The lot's variety, so the publish step can fetch its cover photo to host
+  /// as the offer image. Not published itself — never leaves the device.
+  final String varietyId;
+
   /// What to publish: the variety label (never the full inventory).
   final String summary;
 
@@ -197,7 +202,7 @@ class ShareableLot extends Equatable {
 
   @override
   List<Object?> get props =>
-      [lotId, summary, offerStatus, category, harvestYear, isOrganic];
+      [lotId, varietyId, summary, offerStatus, category, harvestYear, isOrganic];
 }
 
 /// One germination test on a lot; [rate] is derived (0..1), null when it can't
@@ -727,6 +732,12 @@ class VarietyRepository {
     return byVariety;
   }
 
+  /// The cover photo (lowest `sortOrder`) for a single variety, or null when it
+  /// has none. Used by the publish step to host an offer's image; reuses the
+  /// same "first photo" rule as the inventory avatar.
+  Future<Uint8List?> coverPhotoFor(String varietyId) async =>
+      (await _firstPhotosFor([varietyId]))[varietyId];
+
   /// Maps each of [speciesIds] to its scientific name (one query).
   Future<Map<String, String>> _scientificNamesFor(
     Set<String> speciesIds,
@@ -1461,6 +1472,7 @@ class VarietyRepository {
         if (byId[l.varietyId] case final v?)
           ShareableLot(
             lotId: l.id,
+            varietyId: v.id,
             summary: v.label,
             offerStatus: l.offerStatus,
             category: v.category,