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/services/social_service.dart

@@ -39,20 +39,34 @@ class SocialService {
   /// Opens a session against [relayUrls]: one fault-tolerant pool carrying all
   /// three transports (offers, messaging, trust). Relays that are down are
   /// skipped; throws only when none are reachable. Caller closes it.
-  Future<SocialSession> openSession(List<String> relayUrls) async {
+  ///
+  /// [mediaServerUrl] (a Blossom endpoint) enables offer photos: when set, the
+  /// session exposes a [MediaTransport] to host images. It rides HTTP, not the
+  /// relay pool, so it's independent of relay reachability.
+  Future<SocialSession> openSession(
+    List<String> relayUrls, {
+    String? mediaServerUrl,
+  }) async {
     final pool = await RelayPool.connect(relayUrls, identity: identity);
-    return SocialSession(pool);
+    return SocialSession(pool, mediaServerUrl: mediaServerUrl);
   }
 }
 
 /// One relay channel with the three transports on top — the
-/// "one channel, three interfaces" shape, at the app layer.
+/// "one channel, three interfaces" shape, at the app layer. Media rides its own
+/// HTTP endpoint (Blossom), so it's a fourth transport but not on the channel.
 class SocialSession {
-  SocialSession(this._channel)
+  SocialSession(this._channel, {String? mediaServerUrl})
       : offers = NostrOfferTransport(_channel),
         messages = NostrMessageTransport(_channel),
         trust = NostrTrustTransport(_channel),
-        profile = NostrProfileTransport(_channel);
+        profile = NostrProfileTransport(_channel),
+        media = (mediaServerUrl != null && mediaServerUrl.trim().isNotEmpty)
+            ? BlossomMediaTransport(
+                Uri.parse(mediaServerUrl.trim()),
+                _channel.privateKeyHex,
+              )
+            : null;
 
   final NostrChannel _channel;
 
@@ -61,7 +75,14 @@ class SocialSession {
   final TrustTransport trust;
   final ProfileTransport profile;
 
-  Future<void> close() => _channel.close();
+  /// Hosts offer photos, or null when no media server is configured (offers then
+  /// publish without images).
+  final MediaTransport? media;
+
+  Future<void> close() async {
+    await media?.close();
+    await _channel.close();
+  }
 }
 
 Uint8List _hexToBytes(String hex) {

apps/app_seeds/lib/services/social_settings.dart

@@ -17,6 +17,7 @@ class SocialSettings {
   static const _areaKey = 'tane.social.area_geohash';
   static const _relaysKey = 'tane.social.relays';
   static const _searchPrecisionKey = 'tane.social.search_precision';
+  static const _mediaServerKey = 'tane.social.media_server';
 
   /// How wide "your zone" searches, as a geohash prefix length: 5 ≈ ±2.4 km
   /// ("very close"), 4 ≈ ±20 km ("around here"), 3 ≈ ±78 km ("my region").
@@ -39,6 +40,11 @@ class SocialSettings {
     'wss://relay.primal.net',
   ];
 
+  /// Blossom media server used to host offer photos so peers can see them. The
+  /// Comunes-run server is the non-commercial default; the user can point this
+  /// at any Blossom server, or clear it to publish offers without photos.
+  static const String defaultMediaServer = 'https://blossom.comunes.org';
+
   /// The user's coarse area as a low-precision geohash (may be empty). Set
   /// manually or from device location; the transport coarsens it further.
   Future<String> areaGeohash() async => (await _store.read(_areaKey)) ?? '';
@@ -73,6 +79,18 @@ class SocialSettings {
   Future<void> setSearchPrecision(int precision) =>
       _store.write(_searchPrecisionKey, '${_clampPrecision(precision)}');
 
+  /// The Blossom media server for offer photos. Falls back to
+  /// [defaultMediaServer] until the user configures one; an explicitly saved
+  /// empty value turns photo hosting off (offers still publish, without images).
+  Future<String> mediaServerUrl() async {
+    final raw = await _store.read(_mediaServerKey);
+    if (raw == null) return defaultMediaServer; // never configured
+    return raw.trim();
+  }
+
+  Future<void> setMediaServerUrl(String url) =>
+      _store.write(_mediaServerKey, url.trim());
+
   int _clampPrecision(int p) => p < minSearchPrecision
       ? minSearchPrecision
       : (p > maxSearchPrecision ? maxSearchPrecision : p);