Add Stripe payments submodule

[bradleyshep]

Description of Changes

Adds a Stripe submodule that mirrors customers, subscriptions, Checkout sessions, invoices, and payments into private SpacetimeDB tables.

• Synchronous procedures for Stripe billing and query operations.
• Validated webhook ingestion, signature checks, replay, and a host HTTP handler.
• Host-owned views and workflows demonstrated by a store example.
• Package documentation, unit tests, provider smoke scripts, and generated client bindings.

Builds on bradley/submodules-core for crypto and shared example support. It does not depend on the email or agents branches.

Example screenshots

Existing example screenshots from #5823:

Stripe

API and ABI breaking changes

No existing SpacetimeDB API or ABI is changed. This adds new TypeScript package APIs that need review before merge.

Rollback safety impact

n/a. This adds opt-in TypeScript packages and examples; it does not change existing server storage formats.

Expected complexity level and risk

3/5

An isolated, opt-in Stripe integration that does not change existing server behavior. Applications that use it need correct payment authorization, request idempotency, and webhook handling to keep Stripe and local records consistent.

Testing

Verified locally on bradley/submodules-payments:

• Frozen-lockfile install for the root, SDK, and present submodule workspaces.
• Type checks for the added packages.
• Existing test scripts for the added packages and examples.
• Build scripts for the added packages and examples.
• Regenerate client bindings and verify no tracked changes.
• Lint and formatting checks for the added packages and examples.
• Verify workspace dependencies are present in this branch.
• Review the public billing API, authorization, and webhook state transitions.
• Run the Stripe smoke and end-to-end scripts with test credentials and a local SpacetimeDB instance.

Commands used for this group:

pnpm -r -F "./spacetime-stripe-ts/**" run typecheck
pnpm -r -F "./spacetime-stripe-ts/**" run test
pnpm -r -F "./spacetime-stripe-ts/**" run build
pnpm -r -F "./spacetime-stripe-ts/**" run lint

The TypeScript SDK was built first. Live deployment and provider tests were not run during split validation. Existing workspace peer-dependency warnings remain.