Add Resend transactional email submodule

[bradleyshep]

Description of Changes

Adds a Resend submodule for sending transactional email and tracking delivery state in SpacetimeDB.

• Synchronous procedures for outbound Resend requests and email operations.
• Webhook signature verification, payload validation, idempotent ingestion, and replay.
• Private delivery state and administrative configuration.
• Package documentation, unit tests, and a dispatch example with generated client bindings.

Builds on bradley/submodules-core. The package uses crypto; the example also uses rate-limit and shared example support.

Example screenshots

Existing example screenshots from #5823:

Resend

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

2/5

An isolated, opt-in Resend integration that does not change existing server behavior. For applications that use it, review delivery idempotency, webhook verification, and access to provider credentials.

Testing

Verified locally on bradley/submodules-email:

• 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 email API, authorization, and webhook handling.
• Run the Resend smoke test with credentials and a local SpacetimeDB instance.

Commands used for this group:

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