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/test/ui/variety_detail_screen_test.dart

@@ -329,4 +329,33 @@ void main() {
     expect(find.text('Maíz'), findsNothing);
     await disposeTree(tester);
   });
+
+  testWidgets('adding and removing an external link', (tester) async {
+    final repo = newTestRepository(db);
+    final id = await repo.addQuickVariety(label: 'Maize');
+
+    await tester.pumpWidget(
+      wrapDetail(
+        repository: repo,
+        varietyId: id,
+        species: newTestSpeciesRepository(db),
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    await tester.tap(find.byKey(const Key('link.add')));
+    await tester.pumpAndSettle();
+    await tester.enterText(find.byKey(const Key('link.url')), 'wikipedia.org');
+    await tester.enterText(find.byKey(const Key('link.title')), 'Wiki');
+    await tester.tap(find.byKey(const Key('link.save')));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Wiki'), findsOneWidget);
+
+    await tester.tap(find.byIcon(Icons.close)); // the link chip's delete icon
+    await tester.pumpAndSettle();
+
+    expect(find.text('Wiki'), findsNothing);
+    await disposeTree(tester);
+  });
 }