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/ai/ai-trace.service.spec.ts

@@ -143,6 +143,71 @@ describe('AiTraceService receipt masking', () => {
     expect(result.rawOutput).toContain('{"ok":true}');
     expect(result.retryCount).toBe(2);
     expect(result.chunkCount).toBe(4);
-    expect(result.warnings).toContain('parse:low_confidence');
+    expect(result.warnings).toEqual(
+      expect.arrayContaining([
+        expect.objectContaining({
+          kind: 'parse',
+          code: 'low_confidence',
+          title: 'Låg parsningskvalitet',
+          severity: 'warning',
+        }),
+      ]),
+    );
+    expect(result.legacyWarnings).toContain('parse:low_confidence');
+  });
+
+  it('keeps multiple token_overlap warnings for same row', async () => {
+    prismaMock.flyerSession.findUnique.mockResolvedValue({
+      id: 202,
+      userId: 9,
+      createdAt: new Date('2026-05-23T09:00:00.000Z'),
+      sourceFileName: 'willys-v21.pdf',
+      sourceMimeType: 'application/pdf',
+      sourceFileSize: 2222,
+      user: { username: 'admin', email: 'admin@example.com' },
+      items: [
+        {
+          id: 11,
+          rawName: 'Tomatmix',
+          normalizedName: 'tomatmix',
+          brand: null,
+          categoryHint: 'Grönsaker',
+          categoryId: null,
+          price: null,
+          priceUnit: null,
+          comparisonPrice: null,
+          comparisonUnit: null,
+          weight: null,
+          bundleWeight: null,
+          isBundle: false,
+          bundleItems: [],
+          offerText: null,
+          parseConfidence: 0.9,
+          parseReasons: [],
+          matchedProductId: null,
+          matchedProductName: null,
+          matchedVia: 'token',
+          matchConfidence: 0.7,
+          matchReasons: ['token_overlap:0.42', 'token_overlap:0.73'],
+        },
+      ],
+    });
+
+    prismaMock.aiTrace.findMany.mockResolvedValue([
+      {
+        sessionId: 202,
+        prompt: 'prompt',
+        rawOutput: '{"ok":true}',
+        normalizedOutput: { retryCount: 0, chunkCount: 1 },
+      },
+    ]);
+
+    const result = await service.getTraceById('flyer-202');
+
+    const tokenWarnings = result.warnings.filter((warning) => warning.code === 'token_overlap');
+    expect(tokenWarnings).toHaveLength(2);
+    expect(result.legacyWarnings).toEqual(
+      expect.arrayContaining(['match:token_overlap:0.42', 'match:token_overlap:0.73']),
+    );
   });
 });