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>

lib/mix/tasks/generate_mockups.ex

@@ -32,7 +32,7 @@ defmodule Mix.Tasks.GenerateMockups do
 
   use Mix.Task
 
-  alias SimpleshopTheme.Printify.MockupGenerator
+  alias SimpleshopTheme.Mockups.Generator, as: MockupGenerator
 
   @shortdoc "Generates product mockups using Printify API"