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/data/variety_repository.dart

@@ -455,6 +455,42 @@ class VarietyRepository {
     return id;
   }
 
+  /// Updates an existing lot's fields (LWW). The [quantity] and [harvestYear]
+  /// are set as given (null clears them).
+  Future<void> updateLot({
+    required String lotId,
+    required LotType type,
+    int? harvestYear,
+    Quantity? quantity,
+    String? storageLocation,
+  }) async {
+    final (_, updated) = _stamp();
+    await (_db.update(_db.lots)..where((l) => l.id.equals(lotId))).write(
+      LotsCompanion(
+        type: Value(type),
+        harvestYear: Value(harvestYear),
+        quantityKind: Value(quantity?.kind.name),
+        quantityPrecise: Value(quantity?.count?.toDouble()),
+        quantityLabel: Value(quantity?.label),
+        storageLocation: Value(storageLocation),
+        updatedAt: Value(updated),
+        lastAuthor: Value(nodeId),
+      ),
+    );
+  }
+
+  /// Soft-deletes a lot (tombstone); it leaves the variety's lot list.
+  Future<void> softDeleteLot(String lotId) async {
+    final (_, updated) = _stamp();
+    await (_db.update(_db.lots)..where((l) => l.id.equals(lotId))).write(
+      LotsCompanion(
+        isDeleted: const Value(true),
+        updatedAt: Value(updated),
+        lastAuthor: Value(nodeId),
+      ),
+    );
+  }
+
   /// Soft-deletes a variety (tombstone); it disappears from the inventory but
   /// the row survives for correct CRDT merges later.
   Future<void> softDeleteVariety(String id) async {