fix(#24): Implement idempotency protection for critical write operations

backend/migrations/002_idempotency_table.sql

@@ -0,0 +1,9 @@
+-- Migration: 002_idempotency_table
+-- Description: Add idempotency_keys table for handling idempotent requests
+
+CREATE TABLE IF NOT EXISTS idempotency_keys (
+  id BIGINT PRIMARY KEY AUTO_INCREMENT,
+  key VARCHAR(255) NOT NULL UNIQUE,
+  response_body TEXT NULL,
+  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);