feat: add Shopify integration

[Empreiteiro]

Summary

• Add Shopify as a new data source with 8 tables (orders, customers, products, variants, line_items, collections, inventory_items, refunds) and 5 report templates (Sales Overview, Product Performance, Customer Analytics, Inventory Health, Discounts & Refunds)
• Backend: ask_shopify.py script with Shopify Admin API integration using Link header pagination, shopify_router.py with test-connection/discover/refresh-metadata endpoints
• Frontend: ShopifySourceForm.tsx with store name + access token inputs, connection testing with shop info display, and resource count discovery

Test plan

• Verify npm run typecheck passes (confirmed)
• Test connection with a Shopify Custom App access token
• Verify store discovery shows correct resource counts
• Connect a Shopify source and ask questions about orders/products
• Verify all 5 report templates load correctly
• Check i18n strings render correctly in EN, PT, and ES

Closes #71

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