Implement order processing workflow

Build complete order processing workflow with state machine.

Order States:

• PENDING → CONFIRMED → PROCESSING → SHIPPED → DELIVERED
• Error states: CANCELLED, FAILED, REFUNDED

Implementation:

• Design state machine
• Implement state transitions with validation
• Add idempotency for API calls
• Implement order validation rules
• Add inventory checks
• Integrate payment processing
• Add order notifications (email/SMS)

Business Rules:

• Orders can only be cancelled before PROCESSING
• Refunds allowed for DELIVERED orders within 30 days
• Automatic cancellation after 24h if not confirmed