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/data/variety_detail_test.dart

@@ -4,6 +4,7 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:tane/data/species_repository.dart';
 import 'package:tane/data/variety_repository.dart';
 import 'package:tane/db/database.dart';
+import 'package:tane/db/enums.dart';
 
 import '../support/test_support.dart';
 
@@ -139,4 +140,35 @@ void main() {
 
     await queue.cancel();
   });
+
+  test('updateLot changes a lot\'s year, type and quantity', () async {
+    final id = await repo.addQuickVariety(label: 'Maize');
+    final lotId = await repo.addLot(
+      varietyId: id,
+      harvestYear: 2020,
+      quantity: const Quantity(kind: QuantityKind.packet, count: 1),
+    );
+
+    await repo.updateLot(
+      lotId: lotId,
+      type: LotType.plant,
+      harvestYear: 2024,
+      quantity: const Quantity(kind: QuantityKind.plant, count: 3),
+    );
+
+    final lot = (await repo.watchVariety(id).first)!.lots.single;
+    expect(lot.type, LotType.plant);
+    expect(lot.harvestYear, 2024);
+    expect(lot.quantity?.kind, QuantityKind.plant);
+    expect(lot.quantity?.count, 3);
+  });
+
+  test('softDeleteLot removes the lot from the variety', () async {
+    final id = await repo.addQuickVariety(label: 'Maize');
+    final lotId = await repo.addLot(varietyId: id, harvestYear: 2024);
+
+    await repo.softDeleteLot(lotId);
+
+    expect((await repo.watchVariety(id).first)!.lots, isEmpty);
+  });
 }