feat(platform): add webhooks module to the server entrypoint

[ozsay]

Stacked on #316 (base is that branch; retarget to main once it merges).

Summary

• platform.webhooks.verifyEvent(rawBody, headers, { publicKeys, toleranceSeconds? }) verifies Standard Webhooks v1a (Ed25519) signatures with WebCrypto only, so it runs on Node 20+, Deno, Bun and edge runtimes. Returns a typed WebhookEvent; nothing is parsed before the signature passes. Makes no network calls.
• platform.webhooks.createHandler({ publicKeys, on, onEvent, onSuccess, onError, toleranceSeconds? }) returns a fetch-style (request: Request) => Promise<Response>:
  • reads the raw body, verifies, and answers the webhook.test.v1 activation challenge itself (callbacks never see the probe);
  • runs onEvent, then the per-type handler from on (event narrowed per key), then onSuccess;
  • responds 204 on success (including types without a handler), 400 on a verification failure, 500 on misconfigured keys or a throwing callback so Base44 retries;
  • onSuccess / onError may return a Response to override the default.
• Errors: WebhookVerificationError (400) vs WebhookConfigurationError (missing/malformed keys, 500).
• Configuration: createBase44PlatformServer takes only the shared serverUrl / apiKey; each module handles its own settings. The webhooks module takes publicKeys on every call that verifies (verifyEvent, createHandler); there is no module-level init/config. No env reads, no key fetching.
• Types: per-event data interfaces and WebhookEventMap are generated from the schemas published at https://base44.com/webhooks/schemas by npm run generate:webhooks (event list in scripts/webhook-event-types.json; --from <dir> for a local apper checkout).
• Adds engines.node >=20, README section, two examples (verifyEvent + switch, and createHandler), and json-schema-to-typescript as a devDependency.

Test plan

• npm test -w @base44/platform (type tests incl. per-handler narrowing, 63 unit tests incl. tamper/relabel/stale/rotation/config and handler status/callback ordering, typedoc, packed-package runtime and declaration checks)
• npx eslint packages/platform

🤖 Generated with Claude Code

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🚀 Package Preview Available! (@base44/platform)

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Install this PR's preview build with npm:

npm i @base44-preview/platform@0.1.0-pr.319.3e74a61

Prefer not to change any import paths? Install using npm alias so your code still imports @base44/platform:

npm i "@base44/platform@npm:@base44-preview/platform@0.1.0-pr.319.3e74a61"

Or add it to your package.json dependencies:

{
  "dependencies": {
    "@base44/platform": "npm:@base44-preview/platform@0.1.0-pr.319.3e74a61"
  }
}

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