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/flyer-import.service.ts

@@ -9,14 +9,15 @@ import {
 import { Prisma } from '@prisma/client';
 import { PrismaService } from '../prisma/prisma.service';
 import { normalizeName } from '../common/utils/normalize-name';
-import {
-  FlyerImportItem,
-  FlyerImportMatchVia,
-  FlyerImportResponse,
-} from './dto/flyer-import.response';
+import {
+  FlyerImportItem,
+  FlyerImportMatchVia,
+  FlyerImportResponse,
+} from './dto/flyer-import.response';
 import { TextExtractorService } from './services/text-extractor.service';
 import { AiFlyerParserService } from './services/ai-flyer-parser.service';
-import { FlyerNormalizerService } from './services/flyer-normalizer.service';
+import { FlyerNormalizerService } from './services/flyer-normalizer.service';
+import { describeMatchReason, describeParseReason } from './services/reason-codes';
 
 type FlyerParseItem = {
   rawName: string;
@@ -135,13 +136,15 @@ export class FlyerImportService {
         offerLimitText,
         parseConfidence: item.confidence,
         parseReasons: item.reasonCodes,
-        matchedProductId: match.product?.id ?? null,
-        matchedProductName: match.product?.name ?? null,
-        matchedVia: match.via,
-        matchConfidence: match.confidence,
-        matchReasons: match.reasons,
-      };
-    });
+        parseReasonsDetailed: this.describeParseReasons(item.reasonCodes),
+        matchedProductId: match.product?.id ?? null,
+        matchedProductName: match.product?.name ?? null,
+        matchedVia: match.via,
+        matchConfidence: match.confidence,
+        matchReasons: match.reasons,
+        matchReasonsDetailed: this.describeMatchReasons(match.reasons),
+      };
+    });
 
     const persistedItems = await this.persistSessionWithItems(userId, parsed.retailer, items, file);
 
@@ -790,14 +793,24 @@ export class FlyerImportService {
       offerLimitText,
       parseConfidence: item.parseConfidence,
       parseReasons: toStringArray(item.parseReasons),
+      parseReasonsDetailed: this.describeParseReasons(toStringArray(item.parseReasons)),
       matchedProductId: item.matchedProductId,
       matchedProductName: item.matchedProductName,
       matchedVia: normalizedMatchVia,
       matchConfidence: item.matchConfidence ?? 0,
       matchReasons: toStringArray(item.matchReasons),
+      matchReasonsDetailed: this.describeMatchReasons(toStringArray(item.matchReasons)),
     };
   }
 
+  private describeParseReasons(codes: string[]) {
+    return codes.map((code) => describeParseReason(code));
+  }
+
+  private describeMatchReasons(codes: string[]) {
+    return codes.map((code) => describeMatchReason(code));
+  }
+
   private buildCategoryPath(categoryRef?: {
     name: string;
     parent?: {