Sponsorship: Stripe Checkout endpoint (Phase 7.2)

[pablo-clueless]

Scope

POST /sponsorship/checkout — anonymous (sponsors are not members). Creates a Stripe Checkout Session and returns its URL for redirect.

Request

{ "tierId": "…", "billingPeriod": "monthly|annual", "orgName": "…", "contactEmail": "…" }

or, for a custom one-off contribution:

{ "customAmountCents": 50000, "currency": "usd", "orgName": "…", "contactEmail": "…" }

Rules

• Tier path: mode=subscription with the tier's monthly/annual price id. Custom path: mode=payment with an ad-hoc price-from-amount (sane min/max bounds).
• customer_creation=always; prefill org name + email; success URL → {frontend}/sponsors/thanks, cancel URL → /sponsors.
• No Sponsor row is created here — activation happens in the webhook ticket {{S3}} on checkout.session.completed (pass tierId/orgName through Checkout metadata).
• Endpoint is unauthenticated → validate inputs hard and rate-limit-friendly (no DB writes, no email sends).

Error codes

sponsorship.tier_not_found, sponsorship.invalid_amount, sponsorship.invalid_request

Dependencies

Blocked by #14 (tiers + price ids).

Acceptance criteria

• Subscription session built with correct price id per billing period
• Custom amount bounds enforced
• Metadata carries everything {{S3}} needs to build the Sponsor
• Fake-Stripe tests for both modes

Conventions (project-wide, non-negotiable)

• .NET 9, records for immutable shapes, file-scoped namespaces, primary constructors where they read well. Minimal-API endpoints grouped per module via IEndpointModule.MapEndpoints.
• Result<T> (SharedKernel) instead of exception-driven control flow. Endpoint results map failures to ProblemDetails with the stable error codes listed above — the frontend keys off them.
• Module owns its EF Core DbContext mapped to its own Postgres schema. Modules never reference each other's internals — cross-module needs go through a public contract interface or an in-process domain event (IEventPublisher).
• All external calls (GitHub, Stripe, Brevo, Cloudinary, Meilisearch) behind interfaces owned by the consuming module.
• Every list endpoint paginated (offset is fine). xUnit tests in tests/CommunityPro.Tests/<Module>/ following the existing harness patterns (see Members/MembersTestHarness.cs).