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>

mix.exs

@@ -69,7 +69,9 @@ defmodule SimpleshopTheme.MixProject do
       {:bandit, "~> 1.5"},
       {:tidewave, "~> 0.5", only: :dev},
       {:image, "~> 0.54"},
-      {:oban, "~> 2.18"}
+      {:oban, "~> 2.18"},
+      {:ex_money, "~> 5.0"},
+      {:ex_money_sql, "~> 1.0"}
     ]
   end