feat: stripe commerce spine — catalog, checkout, link, and gate

[daopunk]

Summary

• Seed Stripe test catalog Price IDs into docs/commerce/stripe-catalog.md and document commerce env
• Apply hosted Checkout Studio params on signal-commerce session create
• Webhook lifecycle coverage (subscription.updated / deleted, invoice.payment_failed + 7-day past_due grace)
• Pay-gated Stripe !link (requires stripe_subscription_id); paid UX; !enable-sigstack as group bind
• Coarse entitlement gate acceptance tests (product commands; FeatureGrant deferred)
• Alpha + Stripe coexistence matrix under ENTITLEMENTS__ENFORCE

Test plan

• cargo test -p signal-commerce
• cargo test -p signal-bot --lib commands::link entitlement_gate coexist
• Dual-SKU smoke against real Stripe test Checkout URLs
• CI green on this PR

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