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>

config/config.exs

@@ -80,6 +80,9 @@ config :logger, :default_formatter,
 # Use Jason for JSON parsing in Phoenix
 config :phoenix, :json_library, Jason
 
+# ex_money configuration for currency handling
+config :ex_money, default_cldr_backend: SimpleshopTheme.Cldr
+
 # Oban configuration for background jobs
 config :simpleshop_theme, Oban,
   engine: Oban.Engines.Lite,
@@ -87,7 +90,7 @@ config :simpleshop_theme, Oban,
   plugins: [
     {Oban.Plugins.Pruner, max_age: 60}
   ],
-  queues: [images: 2]
+  queues: [images: 2, sync: 1]
 
 # Import environment specific config. This must remain at the bottom
 # of this file so it overrides the configuration defined above.