feat(inventory): external links on the variety detail

Add Wikipedia/forum-style URLs to a variety:
- Repository: addExternalLink/removeExternalLink (soft delete); watchVariety
  loads links and re-emits on external_links changes; ExternalLinkItem model
  (title preferred over URL for display).
- Cubit: addExternalLink/removeExternalLink.
- Detail: a "Links" section with tappable link chips (open via url_launcher,
  http(s):// prepended if missing), delete per chip, and an "Add link" dialog
  (URL + optional title).

Tests: repo add/remove + widget add/remove. 52 green; Linux runs.

apps/app_seeds/lib/data/variety_repository.dart

@@ -126,6 +126,22 @@ class VarietyPhoto extends Equatable {
   List<Object?> get props => [id, bytes];
 }
 
+/// An external URL attached to a variety (Wikipedia, a forum thread…).
+class ExternalLinkItem extends Equatable {
+  const ExternalLinkItem({required this.id, required this.url, this.title});
+
+  final String id;
+  final String url;
+  final String? title;
+
+  /// What to show: the title if present, otherwise the URL.
+  String get display =>
+      (title != null && title!.trim().isNotEmpty) ? title! : url;
+
+  @override
+  List<Object?> get props => [id, url, title];
+}
+
 /// The full detail of one variety (identity + its lots, names and photos).
 class VarietyDetail extends Equatable {
   const VarietyDetail({
@@ -138,6 +154,7 @@ class VarietyDetail extends Equatable {
     this.lots = const [],
     this.vernacularNames = const [],
     this.photos = const [],
+    this.links = const [],
   });
 
   final String id;
@@ -149,6 +166,7 @@ class VarietyDetail extends Equatable {
   final List<VarietyLot> lots;
   final List<VernacularName> vernacularNames;
   final List<VarietyPhoto> photos;
+  final List<ExternalLinkItem> links;
 
   @override
   List<Object?> get props => [
@@ -161,6 +179,7 @@ class VarietyDetail extends Equatable {
     lots,
     vernacularNames,
     photos,
+    links,
   ];
 }
 
@@ -347,6 +366,9 @@ class VarietyRepository {
       (_db.select(
         _db.attachments,
       )..where((a) => a.parentId.equals(id))).watch().map((_) {}),
+      (_db.select(
+        _db.externalLinks,
+      )..where((e) => e.parentId.equals(id))).watch().map((_) {}),
       // Coarse: any germination-test change re-emits (catalog of tests is tiny).
       _db.select(_db.germinationTests).watch().map((_) {}),
     ]);
@@ -412,6 +434,17 @@ class VarietyRepository {
               ..orderBy([(a) => OrderingTerm(expression: a.createdAt)]))
             .get();
 
+    final links =
+        await (_db.select(_db.externalLinks)
+              ..where(
+                (e) =>
+                    e.parentId.equals(id) &
+                    e.parentType.equalsValue(ParentType.variety) &
+                    e.isDeleted.equals(false),
+              )
+              ..orderBy([(e) => OrderingTerm(expression: e.createdAt)]))
+            .get();
+
     return VarietyDetail(
       id: v.id,
       label: v.label,
@@ -434,6 +467,9 @@ class VarietyRepository {
         for (final p in photos)
           if (p.bytes != null) VarietyPhoto(id: p.id, bytes: p.bytes!),
       ],
+      links: links
+          .map((e) => ExternalLinkItem(id: e.id, url: e.url, title: e.title))
+          .toList(),
     );
   }
 
@@ -630,6 +666,45 @@ class VarietyRepository {
     );
   }
 
+  /// Adds an external link (URL, optional title) to a variety. Returns its id.
+  Future<String> addExternalLink(
+    String varietyId,
+    String url, {
+    String? title,
+  }) async {
+    final (created, updated) = _stamp();
+    final id = idGen.newId();
+    await _db
+        .into(_db.externalLinks)
+        .insert(
+          ExternalLinksCompanion.insert(
+            id: id,
+            createdAt: created,
+            updatedAt: updated,
+            lastAuthor: nodeId,
+            parentType: ParentType.variety,
+            parentId: varietyId,
+            url: url,
+            title: Value(title),
+          ),
+        );
+    return id;
+  }
+
+  /// Soft-deletes an external link (tombstone).
+  Future<void> removeExternalLink(String linkId) async {
+    final (_, updated) = _stamp();
+    await (_db.update(
+      _db.externalLinks,
+    )..where((e) => e.id.equals(linkId))).write(
+      ExternalLinksCompanion(
+        isDeleted: const Value(true),
+        updatedAt: Value(updated),
+        lastAuthor: Value(nodeId),
+      ),
+    );
+  }
+
   /// Soft-deletes a variety (tombstone); it disappears from the inventory but
   /// the row survives for correct CRDT merges later.
   Future<void> softDeleteVariety(String id) async {