feat(ai): enhance AI trace warnings and reason codes system

- Added structured warning system with `AdminAiWarning` type in backend and Flutter
- Implemented detailed reason descriptors with `FlyerReasonDescriptor` for parse and match operations
- Added `legacyWarnings` field to maintain backward compatibility
- Enhanced AI trace service to collect and format warnings with item-level context
- Updated flyer import services to include detailed reason descriptions in responses
- Added Swedish diacritic preservation for cheese variants (Prästost, Herrgårdsost, Grevéost)
- Implemented UTF-8 content validation for AI responses
- Added new reason code definitions in `reason-codes.ts`
- Updated Flutter UI to display structured warnings with severity indicators
- Added error report generation and copy functionality in admin panel
- Added comprehensive test coverage for new warning system and cheese normalization

BREAKING CHANGE: AI trace warnings are now structured objects instead of simple strings

backend/src/flyer-import/dto/flyer-import.response.ts

@@ -1,4 +1,13 @@
-export type FlyerImportMatchVia = 'alias' | 'exact' | 'token' | 'none';
+export type FlyerImportMatchVia = 'alias' | 'exact' | 'token' | 'none';
+
+export type FlyerReasonDescriptor = {
+  code: string;
+  kind: 'parse' | 'match';
+  title: string;
+  message: string;
+  severity: 'info' | 'warning' | 'error';
+  location: string | null;
+};
 
 export type FlyerImportItem = {
   flyerItemId: number | null;
@@ -18,14 +27,16 @@ export type FlyerImportItem = {
   offerText: string | null;
   isOffer: boolean;
   offerLimitText: string | null;
-  parseConfidence: number;
-  parseReasons: string[];
-  matchedProductId: number | null;
-  matchedProductName: string | null;
-  matchedVia: FlyerImportMatchVia;
-  matchConfidence: number;
-  matchReasons: string[];
-};
+  parseConfidence: number;
+  parseReasons: string[];
+  parseReasonsDetailed: FlyerReasonDescriptor[];
+  matchedProductId: number | null;
+  matchedProductName: string | null;
+  matchedVia: FlyerImportMatchVia;
+  matchConfidence: number;
+  matchReasons: string[];
+  matchReasonsDetailed: FlyerReasonDescriptor[];
+};
 
 export type FlyerImportResponse = {
   sessionId: number | null;