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/test/ui/variety_detail_screen_test.dart

@@ -200,4 +200,59 @@ void main() {
     expect(find.text('90%'), findsOneWidget); // germination badge on the lot
     await disposeTree(tester);
   });
+
+  testWidgets('editing a lot updates its line', (tester) async {
+    final repo = newTestRepository(db);
+    final id = await repo.addQuickVariety(label: 'Maize');
+    await repo.addLot(
+      varietyId: id,
+      harvestYear: 2020,
+      quantity: const Quantity(kind: QuantityKind.packet, count: 1),
+    );
+
+    await tester.pumpWidget(
+      wrapDetail(
+        repository: repo,
+        varietyId: id,
+        species: newTestSpeciesRepository(db),
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    await tester.tap(find.text('Year 2020 · 1 packet')); // open the lot
+    await tester.pumpAndSettle();
+
+    await tester.tap(find.byKey(const Key('addLot.year')));
+    await tester.pumpAndSettle();
+    await tester.tap(find.text('2024').last);
+    await tester.pumpAndSettle();
+    await tester.tap(find.byKey(const Key('addLot.save')));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Year 2024 · 1 packet'), findsOneWidget);
+    await disposeTree(tester);
+  });
+
+  testWidgets('deleting a lot removes it', (tester) async {
+    final repo = newTestRepository(db);
+    final id = await repo.addQuickVariety(label: 'Maize');
+    await repo.addLot(varietyId: id, harvestYear: 2020);
+
+    await tester.pumpWidget(
+      wrapDetail(
+        repository: repo,
+        varietyId: id,
+        species: newTestSpeciesRepository(db),
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    await tester.tap(find.textContaining('Year 2020')); // open the lot
+    await tester.pumpAndSettle();
+    await tester.tap(find.byKey(const Key('lot.delete')));
+    await tester.pumpAndSettle();
+
+    expect(find.text('No lots yet.'), findsOneWidget);
+    await disposeTree(tester);
+  });
 }