umbrella: pet-store storefront fixes (rolling)

Summary

Second umbrella round on top of the first one (PR #2, squash-merged as d69a2e9 on 2026-05-08). Same rolling-PR rule: all in-flight pet-store work commits onto umbrella/pet-store; this PR stays open and accumulates commits until the user squash-merges into main.

Tracking issue: #1.

In this round so far

• feat(commerce): shipping notification email + admin mark-shipped action — adds orders.{tracking_number, carrier, tracking_url, shipped_at}, an order-shipped.stx email template, an admin-gated POST /api/orders/{id}/ship action that updates the row + fires the email, and a tracking-aware tile on /orders/{uuid}.

Test plan

• Run the new migration: ./buddy migrate
• Place a test order via the storefront flow
• As an admin (dashboard session), POST /api/orders/<id>/ship with tracking_number=1Z999...&carrier=UPS&tracking_url=https://wwwapps.ups.com/...
• Confirm the customer's email arrives with carrier + tracking link
• Confirm /orders/<uuid> shows the updated "Shipped on …" tile with the tracking link

🤖 Generated with Claude Code