feat(inventory): edit and delete existing lots

Lots could only be created; now they can be edited and removed:
- Repository: updateLot (LWW) + softDeleteLot (tombstone).
- Cubit: updateLot / deleteLot.
- The add-lot sheet is now a shared add/edit sheet — tapping a lot opens it
  prefilled (type, year, quantity), with a trash action to delete. The reactive
  detail view refreshes on save/delete.

Tests: repo updateLot/softDeleteLot + widget edit-lot and delete-lot. 42 green.

apps/app_seeds/lib/state/variety_detail_cubit.dart

@@ -62,6 +62,20 @@ class VarietyDetailCubit extends Cubit<VarietyDetailState> {
     quantity: quantity,
   );
 
+  Future<void> updateLot({
+    required String lotId,
+    required LotType type,
+    int? year,
+    Quantity? quantity,
+  }) => _repo.updateLot(
+    lotId: lotId,
+    type: type,
+    harvestYear: year,
+    quantity: quantity,
+  );
+
+  Future<void> deleteLot(String lotId) => _repo.softDeleteLot(lotId);
+
   Future<void> linkSpecies(String speciesId) =>
       _repo.linkSpecies(varietyId, speciesId);