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

flutter/lib/features/admin/domain/admin_ai_trace_detail.dart

@@ -1,5 +1,53 @@
 import 'admin_ai_trace.dart';
 
+class AdminAiWarning {
+  final String code;
+  final String kind;
+  final String title;
+  final String message;
+  final String severity;
+  final String? location;
+  final int? itemIndex;
+
+  const AdminAiWarning({
+    required this.code,
+    required this.kind,
+    required this.title,
+    required this.message,
+    required this.severity,
+    required this.location,
+    required this.itemIndex,
+  });
+
+  factory AdminAiWarning.fromJson(Map<String, dynamic> json) {
+    return AdminAiWarning(
+      code: (json['code'] ?? '').toString(),
+      kind: (json['kind'] ?? '').toString(),
+      title: (json['title'] ?? '').toString(),
+      message: (json['message'] ?? '').toString(),
+      severity: (json['severity'] ?? '').toString(),
+      location: json['location']?.toString(),
+      itemIndex: (json['itemIndex'] as num?)?.toInt(),
+    );
+  }
+
+  factory AdminAiWarning.fromLegacy(String value) {
+    final trimmed = value.trim();
+    final parts = trimmed.split(':');
+    final kind = parts.isNotEmpty ? parts.first : 'parse';
+    final code = parts.length > 1 ? parts.sublist(1).join(':') : trimmed;
+    return AdminAiWarning(
+      code: code,
+      kind: kind,
+      title: trimmed,
+      message: trimmed,
+      severity: 'warning',
+      location: null,
+      itemIndex: null,
+    );
+  }
+}
+
 class AdminAiTraceDetail {
   final String id;
   final AdminAiTraceSource source;
@@ -13,7 +61,8 @@ class AdminAiTraceDetail {
   final int? durationMs;
   final int? retryCount;
   final int? chunkCount;
-  final List<String> warnings;
+  final List<AdminAiWarning> warnings;
+  final List<String> legacyWarnings;
   final String? error;
   final String? prompt;
   final String? rawOutput;
@@ -34,6 +83,7 @@ class AdminAiTraceDetail {
     required this.retryCount,
     required this.chunkCount,
     required this.warnings,
+    required this.legacyWarnings,
     required this.error,
     required this.prompt,
     required this.rawOutput,
@@ -43,6 +93,7 @@ class AdminAiTraceDetail {
 
   factory AdminAiTraceDetail.fromJson(Map<String, dynamic> json) {
     final warningsRaw = (json['warnings'] as List<dynamic>?) ?? const [];
+    final legacyWarningsRaw = (json['legacyWarnings'] as List<dynamic>?) ?? const [];
     final normalizedOutputMap = json['normalizedOutput'] is Map
         ? Map<String, dynamic>.from(json['normalizedOutput'] as Map)
         : null;
@@ -64,7 +115,15 @@ class AdminAiTraceDetail {
       durationMs: (json['durationMs'] as num?)?.toInt(),
       retryCount: (json['retryCount'] as num?)?.toInt(),
       chunkCount: (json['chunkCount'] as num?)?.toInt(),
-      warnings: warningsRaw.map((entry) => entry.toString()).toList(),
+      warnings: warningsRaw
+          .map((entry) {
+            if (entry is Map) {
+              return AdminAiWarning.fromJson(Map<String, dynamic>.from(entry));
+            }
+            return AdminAiWarning.fromLegacy(entry.toString());
+          })
+          .toList(),
+      legacyWarnings: legacyWarningsRaw.map((entry) => entry.toString()).toList(),
       error: json['error']?.toString(),
       prompt: json['prompt']?.toString(),
       rawOutput: json['rawOutput']?.toString(),