fix(webhook): decode request body to string to avoid Buffer in Deno

[matingathani]

Summary

The Stripe SDK calls Buffer.isBuffer() internally when processing the webhook payload. Buffer is a Node.js global that doesn't exist in the Deno edge function runtime, causing:

ReferenceError: Buffer is not defined
  at _StripeSyncWebhook.processWebhook (index.ts:47309:65)

Root cause: stripe-webhook.ts passed a Uint8Array to processWebhook, which forwarded it to the Stripe SDK's constructEventAsync. The SDK's internal signature computation checks Buffer.isBuffer(payload) — in Deno that throws instead of returning false.

Fix: Decode the ArrayBuffer to a UTF-8 string before calling processWebhook. The Stripe signature verification works correctly on strings, and the string code path in the SDK never references Buffer.

Test plan

• Deploy the edge function to a Supabase project and fire a test webhook — verify { received: true } response with no Buffer is not defined error
• Verify signature rejection still works by sending a webhook with a wrong secret

Closes #170

[Copilot]

Pull request overview

Updates the Supabase (Deno) edge webhook handler to avoid a runtime crash caused by the Stripe SDK referencing the Node.js-only Buffer global during webhook signature verification.

Changes:

• Read the webhook body as an ArrayBuffer and decode it to a UTF-8 string.
• Pass the decoded string payload into stripeSync.webhook.processWebhook(...) instead of a Uint8Array.

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