feat(ai): add AI trace tracking and admin panel

- Add AiTrace model to Prisma schema with relations to User
- Implement AiTraceService with CRUD operations for AI traces
- Add new admin panel for AI traces with filtering and detail views
- Integrate trace persistence in receipt import flow
- Add API endpoints for listing and retrieving AI traces
- Update Flutter admin UI with new AI tab and navigation
- Add new domain models for AI traces and details
- Add migration for AiTrace table creation

BREAKING CHANGE: None

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

@@ -0,0 +1,75 @@
+import 'admin_ai_trace.dart';
+
+class AdminAiTraceDetail {
+  final String id;
+  final AdminAiTraceSource source;
+  final AdminAiTraceStatus status;
+  final DateTime createdAt;
+  final int userId;
+  final String userLabel;
+  final int? sessionId;
+  final String? fileName;
+  final String? model;
+  final int? durationMs;
+  final int? retryCount;
+  final int? chunkCount;
+  final List<String> warnings;
+  final String? error;
+  final String? prompt;
+  final String? rawOutput;
+  final Map<String, dynamic>? normalizedOutput;
+  final Map<String, dynamic> summary;
+
+  const AdminAiTraceDetail({
+    required this.id,
+    required this.source,
+    required this.status,
+    required this.createdAt,
+    required this.userId,
+    required this.userLabel,
+    required this.sessionId,
+    required this.fileName,
+    required this.model,
+    required this.durationMs,
+    required this.retryCount,
+    required this.chunkCount,
+    required this.warnings,
+    required this.error,
+    required this.prompt,
+    required this.rawOutput,
+    required this.normalizedOutput,
+    required this.summary,
+  });
+
+  factory AdminAiTraceDetail.fromJson(Map<String, dynamic> json) {
+    final warningsRaw = (json['warnings'] as List<dynamic>?) ?? const [];
+    final normalizedOutputMap = json['normalizedOutput'] is Map
+        ? Map<String, dynamic>.from(json['normalizedOutput'] as Map)
+        : null;
+    final summaryMap = json['summary'] is Map
+        ? Map<String, dynamic>.from(json['summary'] as Map)
+        : const <String, dynamic>{};
+
+    return AdminAiTraceDetail(
+      id: (json['id'] ?? '').toString(),
+      source: AdminAiTraceSourceX.fromApi(json['source']?.toString()),
+      status: AdminAiTraceStatusX.fromApi(json['status']?.toString()),
+      createdAt: DateTime.tryParse((json['createdAt'] ?? '').toString()) ??
+          DateTime.fromMillisecondsSinceEpoch(0),
+      userId: (json['userId'] as num?)?.toInt() ?? 0,
+      userLabel: (json['userLabel'] ?? '').toString(),
+      sessionId: (json['sessionId'] as num?)?.toInt(),
+      fileName: json['fileName']?.toString(),
+      model: json['model']?.toString(),
+      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(),
+      error: json['error']?.toString(),
+      prompt: json['prompt']?.toString(),
+      rawOutput: json['rawOutput']?.toString(),
+      normalizedOutput: normalizedOutputMap,
+      summary: summaryMap,
+    );
+  }
+}