feat: add Printify order submission and fulfilment tracking

Submit paid orders to Printify via provider API with idempotent
guards, Stripe address mapping, and error handling. Track fulfilment
status through submitted → processing → shipped → delivered via
webhook-driven updates (primary) and Oban Cron polling fallback.

- 9 fulfilment fields on orders (status, provider IDs, tracking, timestamps)
- OrderSubmissionWorker with retry logic, auto-enqueued after Stripe payment
- FulfilmentStatusWorker polls every 30 mins for missed webhook events
- Printify order webhook handlers (sent-to-production, shipment, delivered)
- Admin UI: fulfilment column in table, fulfilment card with tracking info,
  submit/retry and refresh buttons on order detail
- Mox provider mocking for test isolation (Provider.for_type configurable)
- 33 new tests (555 total), verified against real Printify API

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

mix.exs

@@ -48,6 +48,7 @@ defmodule SimpleshopTheme.MixProject do
       {:phoenix_html, "~> 4.1"},
       {:phoenix_live_reload, "~> 1.2", only: :dev},
       {:phoenix_live_view, "~> 1.1.0"},
+      {:mox, "~> 1.0", only: :test},
       {:lazy_html, ">= 0.1.0", only: :test},
       {:phoenix_live_dashboard, "~> 0.8.3"},
       {:esbuild, "~> 0.10", runtime: Mix.env() == :dev},