feat: add unit mapping functionality

- Added new API path for unit mappings in `api_paths.dart`.
- Implemented `upsertUnitMapping` method in `ImportRepository` to handle unit mapping creation.
- Updated `ReceiptImportTab` to learn and save unit mappings during receipt import.
- Created DTO for unit mapping with validation in `create-unit-mapping.dto.ts`.
- Added SQL migration for `UnitMapping` table creation with necessary constraints.

flutter/lib/features/import/presentation/receipt_import_tab.dart

@@ -464,8 +464,10 @@ class _ReceiptImportTabState extends ConsumerState<ReceiptImportTab> {
     int pantryAdded = 0;
     int pantrySkipped = 0;
     int aliasesLearned = 0;
+    int unitMappingsLearned = 0;
     try {
       final token = await ref.read(authStateProvider.future);
+      final repo = ref.read(importRepositoryProvider);
       final invRepo = ref.read(inventoryRepositoryProvider);
       final pantryRepo = ref.read(pantryRepositoryProvider);
       final adminRepo = ref.read(adminRepositoryProvider);
@@ -497,6 +499,8 @@ class _ReceiptImportTabState extends ConsumerState<ReceiptImportTab> {
               : (edit.quantity ?? inferred.totalQuantity ?? item.quantity ?? 1.0);
           final unit = packUnit;
           final existing = _inventoryByProduct[pid];
+            final originalUnit = (item.unit ?? '').trim();
+            final preferredUnitForLearning = (existing?.unit ?? unit).trim();
           final qtyInExistingUnit = existing == null
               ? null
               : convertQuantity(qty, unit, existing.unit);
@@ -516,6 +520,23 @@ class _ReceiptImportTabState extends ConsumerState<ReceiptImportTab> {
             }, token: token);
             created++;
           }
+
+          if (originalUnit.isNotEmpty && preferredUnitForLearning.isNotEmpty) {
+            try {
+              await repo.upsertUnitMapping(
+                productId: pid,
+                originalUnit: originalUnit,
+                preferredUnit: preferredUnitForLearning,
+                token: token,
+              );
+              if (originalUnit.toLowerCase().trim() != preferredUnitForLearning.toLowerCase().trim()) {
+                unitMappingsLearned++;
+              }
+            } catch (e, st) {
+              debugPrint('ReceiptImportTab unit mapping upsert failed: $e');
+              debugPrintStack(stackTrace: st);
+            }
+          }
         }
 
         final normalizedReceiptName = item.rawName.trim().toLowerCase();
@@ -544,6 +565,7 @@ class _ReceiptImportTabState extends ConsumerState<ReceiptImportTab> {
         if (pantryAdded > 0) '$pantryAdded tillagd${pantryAdded == 1 ? '' : 'a'} i baslager',
         if (pantrySkipped > 0) '$pantrySkipped fanns redan i baslager',
         if (aliasesLearned > 0) '$aliasesLearned alias inlärda',
+        if (unitMappingsLearned > 0) '$unitMappingsLearned enhetsmappningar inlärda',
       ];
       ScaffoldMessenger.of(context).showSnackBar(
         SnackBar(content: Text(parts.join(', ') + '.')),