Add spa membership checkout flow

[awitherow]

Summary

• add /membership page with custom monthly/yearly stored-value treatment credit amount picker
• add dynamic Stripe Checkout subscription route for spa membership credit plus processing-fee line item
• add /membership/success and header nav link

Terms

• selected membership amount becomes treatment credit toward eligible procedures
• credit applies against current listed prices, subject to change
• processing fees are added separately so selected amount remains credit

Verification

• pnpm lint
• pnpm build
• local /membership HTTP 200 and copy/schema marker check

Not run

• Stripe Checkout Session smoke, intentionally skipped per Austin; existing Stripe product flow is already working and this PR reuses the same Stripe client/env pattern.

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Project	Deployment	Actions	Updated (UTC)
williamsburgmedspa	Ready	Preview, Comment	Jun 20, 2026 5:47pm

[awitherow]

Moved back to draft and held outside the SEO release train. This changes subscription, stored-value credit, and Stripe behavior and is not eligible to merge without a fresh current-main rebase, confirmed membership/product terms, and an end-to-end Stripe Checkout plus webhook/entitlement smoke.

[awitherow]

Membership readiness review at exact SHA 8ed74d021f0668333ab70fc5e758e7e4c188718a:

• pnpm lint: passed with two pre-existing <img> warnings.
• pnpm build: passed.
• Checkout correctly creates a Stripe subscription and attaches membership metadata.
• BLOCK: checkout.session.completed is currently routed to syncOrderFromStripeSession(), but membership Checkout does not set metadata.orderId; the webhook therefore throws Stripe session is missing order metadata.
• BLOCK: recurring renewals are not handled (invoice.paid / equivalent), so new treatment credit is not ledgered per successful billing period.
• BLOCK: there is no idempotent membership balance/entitlement ledger or cancellation/refund lifecycle.

Keep this PR draft. Next safe slice is a tested membership webhook/ledger contract after the client confirms final monthly/yearly credit and fee terms. Do not run a live Checkout until that path is implemented and tested.