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

backend/prisma/migrations/20260521171000_add_ai_trace/migration.sql

@@ -0,0 +1,24 @@
+-- CreateTable
+CREATE TABLE `AiTrace` (
+  `id` INTEGER NOT NULL AUTO_INCREMENT,
+  `source` VARCHAR(191) NOT NULL,
+  `userId` INTEGER NULL,
+  `sessionId` INTEGER NULL,
+  `model` VARCHAR(191) NULL,
+  `prompt` LONGTEXT NULL,
+  `rawOutput` LONGTEXT NULL,
+  `normalizedOutput` JSON NULL,
+  `status` VARCHAR(191) NOT NULL,
+  `error` TEXT NULL,
+  `durationMs` INTEGER NULL,
+  `createdAt` DATETIME(3) NOT NULL DEFAULT CURRENT_TIMESTAMP(3),
+  `updatedAt` DATETIME(3) NOT NULL,
+
+  INDEX `AiTrace_source_createdAt_idx`(`source`, `createdAt`),
+  INDEX `AiTrace_userId_createdAt_idx`(`userId`, `createdAt`),
+  INDEX `AiTrace_status_createdAt_idx`(`status`, `createdAt`),
+  PRIMARY KEY (`id`)
+) DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
+
+-- AddForeignKey
+ALTER TABLE `AiTrace` ADD CONSTRAINT `AiTrace_userId_fkey` FOREIGN KEY (`userId`) REFERENCES `User`(`id`) ON DELETE SET NULL ON UPDATE CASCADE;