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/state/variety_detail_cubit.dart

@@ -92,6 +92,12 @@ class VarietyDetailCubit extends Cubit<VarietyDetailState> {
   Future<void> removePhoto(String attachmentId) =>
       _repo.removePhoto(attachmentId);
 
+  Future<void> addExternalLink(String url, {String? title}) =>
+      _repo.addExternalLink(varietyId, url, title: title);
+
+  Future<void> removeExternalLink(String linkId) =>
+      _repo.removeExternalLink(linkId);
+
   Future<void> linkSpecies(String speciesId) =>
       _repo.linkSpecies(varietyId, speciesId);