Create direct purchase order

Create a direct purchase order from listing

Creates an order directly from a listing with variant selection, bypassing the cart. This implements "Buy Now" functionality for immediate purchases.

Process:

1. Validates listing exists and is active
2. Validates variant exists and belongs to listing
3. Checks variant availability (sufficient stock)
4. Snapshots variant price at purchase time
5. Creates order record in V1 orders table
6. Creates market_orders entry (V1 compatibility)
7. Creates order_market_items_v2 entry with variant tracking
8. Allocates stock from variant-specific lots
9. Returns order details with purchase summary

Uses database transaction for atomicity to ensure all-or-nothing behavior. Stock allocation uses listing_item_lots directly (V2).

Requirements:

• Validate listing exists and is active
• Validate variant exists and belongs to listing
• Check variant availability before order creation
• Create order with variant-specific line item
• Allocate stock for the variant
• Use database transaction for atomicity
• Snapshot variant price at time of purchase
• Return order_id and purchase summary
• Prevent ordering unavailable variants with descriptive error
• Follow same patterns as cart checkout