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>

priv/repo/migrations/20260207234225_add_fulfilment_fields_to_orders.exs

@@ -0,0 +1,20 @@
+defmodule SimpleshopTheme.Repo.Migrations.AddFulfilmentFieldsToOrders do
+  use Ecto.Migration
+
+  def change do
+    alter table(:orders) do
+      add :fulfilment_status, :string, null: false, default: "unfulfilled"
+      add :provider_order_id, :string
+      add :provider_status, :string
+      add :fulfilment_error, :string
+      add :tracking_number, :string
+      add :tracking_url, :string
+      add :submitted_at, :utc_datetime
+      add :shipped_at, :utc_datetime
+      add :delivered_at, :utc_datetime
+    end
+
+    create index(:orders, [:fulfilment_status])
+    create index(:orders, [:provider_order_id])
+  end
+end