feat: add Products context with provider integration (Phase 1)

Implement the schema foundation for syncing products from POD providers
like Printify. This includes encrypted credential storage, product/variant
schemas, and an Oban worker for background sync.

New modules:
- Vault: AES-256-GCM encryption for API keys
- Products context: CRUD and sync operations for products
- Provider behaviour: abstraction for POD provider implementations
- ProductSyncWorker: Oban job for async product sync

Schemas: ProviderConnection, Product, ProductImage, ProductVariant

Also reorganizes Printify client to lib/simpleshop_theme/clients/ and
mockup generator to lib/simpleshop_theme/mockups/ for better structure.

134 tests added covering all new functionality.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

test/simpleshop_theme/sync/product_sync_worker_test.exs

@@ -0,0 +1,69 @@
+defmodule SimpleshopTheme.Sync.ProductSyncWorkerTest do
+  use SimpleshopTheme.DataCase, async: false
+  use Oban.Testing, repo: SimpleshopTheme.Repo
+
+  alias SimpleshopTheme.Sync.ProductSyncWorker
+
+  import SimpleshopTheme.ProductsFixtures
+
+  describe "perform/1" do
+    test "cancels for missing connection" do
+      fake_id = Ecto.UUID.generate()
+
+      assert {:cancel, :connection_not_found} =
+               perform_job(ProductSyncWorker, %{provider_connection_id: fake_id})
+    end
+
+    test "cancels for disabled connection" do
+      conn = provider_connection_fixture(%{enabled: false})
+
+      assert {:cancel, :connection_disabled} =
+               perform_job(ProductSyncWorker, %{provider_connection_id: conn.id})
+    end
+  end
+
+  describe "enqueue/1" do
+    test "creates a job with correct args" do
+      # Temporarily switch to manual mode to avoid inline execution
+      Oban.Testing.with_testing_mode(:manual, fn ->
+        conn = provider_connection_fixture()
+
+        assert {:ok, %Oban.Job{} = job} = ProductSyncWorker.enqueue(conn.id)
+        # In manual mode, args use atom keys
+        assert job.args == %{provider_connection_id: conn.id}
+        assert job.queue == "sync"
+      end)
+    end
+  end
+
+  describe "enqueue/2 with delay" do
+    test "schedules a job for later" do
+      Oban.Testing.with_testing_mode(:manual, fn ->
+        conn = provider_connection_fixture()
+
+        assert {:ok, %Oban.Job{} = job} = ProductSyncWorker.enqueue(conn.id, 60)
+        # In manual mode, args use atom keys
+        assert job.args == %{provider_connection_id: conn.id}
+
+        # Job should be scheduled in the future
+        assert DateTime.compare(job.scheduled_at, DateTime.utc_now()) == :gt
+      end)
+    end
+  end
+
+  describe "job creation" do
+    test "new/1 creates job changeset with provider_connection_id" do
+      changeset = ProductSyncWorker.new(%{provider_connection_id: "test-id"})
+
+      assert changeset.changes.args == %{provider_connection_id: "test-id"}
+      assert changeset.changes.queue == "sync"
+    end
+
+    test "new/2 with scheduled_at creates scheduled job" do
+      future = DateTime.add(DateTime.utc_now(), 60, :second)
+      changeset = ProductSyncWorker.new(%{provider_connection_id: "test-id"}, scheduled_at: future)
+
+      assert changeset.changes.scheduled_at == future
+    end
+  end
+end