[Feature] Printful POD integration — merch shop fulfilment

## Overview
Integrate Printful as the print-on-demand fulfilment backend for the OpenFront merch shop. Stripe checkout triggers Printful order creation automatically.

## Architecture
```
Customer completes checkout
↓
Stripe fires checkout.session.completed webhook
↓
/api/webhooks/stripe maps line items → Printful sync_variant_ids
↓
POST /orders to Printful with idempotency key (Stripe session ID)
↓
Printful fulfils and fires package_shipped webhook
↓
Customer receives tracking email
```

## Key implementation points
- Idempotency via `X-PF-Idempotency-Key: ` — non-negotiable
- Shipping rates fetched from Printful `/shipping/rates` **before** Stripe Checkout is initiated
- SKU → `sync_variant_id` mapping in DB (NOT catalog `variant_id` — different number)
- Stripe Tax enabled for UK (20% VAT), EU (OSS), US (economic nexus)
- Printful wallet must be funded with auto-recharge enabled
- `confirm: true` in order payload to skip draft window

## Acceptance criteria
- [ ] Full flow tested in Printful sandbox
- [ ] Idempotency verified (webhook fires twice → only one order created)
- [ ] Shipping rate correct for UK, EU, and US addresses
- [ ] VAT calculated correctly at checkout for UK customer (20%)
- [ ] Tracking email sent on `package_shipped` webhook
- [ ] `order_failed` alert fires to ops team

## Dependencies
- **Depends on:** Stripe checkout, OAuth
- Stripe Tax must be enabled before launch
- Printful wallet funded with auto-recharge