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>

priv/repo/migrations/20260128235847_create_product_images.exs

@@ -0,0 +1,18 @@
+defmodule SimpleshopTheme.Repo.Migrations.CreateProductImages do
+  use Ecto.Migration
+
+  def change do
+    create table(:product_images, primary_key: false) do
+      add :id, :binary_id, primary_key: true
+      add :product_id, references(:products, type: :binary_id, on_delete: :delete_all), null: false
+      add :src, :string, null: false
+      add :position, :integer, default: 0, null: false
+      add :alt, :string
+
+      timestamps(type: :utc_datetime)
+    end
+
+    # Composite index covers queries on product_id alone (leftmost prefix)
+    create index(:product_images, [:product_id, :position])
+  end
+end