feat: alpha coexistence with Stripe link and paid gating

[daopunk]

Parent: #68

Summary

Alpha redeem alongside Stripe Checkout link_tokens and paid gating. Runs after the checkout / webhook / Stripe !link spine — not before.

Scope

• !link matches pending Stripe tokens and alpha codes without colliding
• Coexistence with paid entitlements (composition already prefers stripe on overlap)
• Any remaining ops UX (list/revoke batches) beyond the fast-path mint CLI
• Interactions with entitlement enforcement when gating ships

Depends on

• Checkout session + webhooks + Stripe !link bind
• Fast alpha redeem (standalone) for the alpha-only store/handler baseline

Out of scope

• Static-site-only alpha knock-out (separate standalone issue)