fix(stripe): use Stripe webhook signatures

[EfeDurmaz16]

Summary

Fixes Stripe webhook signing for configured Stripe webhooks.

Stripe seeded webhooks now send a Stripe-compatible Stripe-Signature header (t=<timestamp>,v1=<hmac>) instead of the GitHub-style X-Hub-Signature-256 header. GitHub-style webhook subscriptions remain the default behavior for existing GitHub webhook flows.

Details

• Adds an optional webhook signatureScheme to the shared dispatcher.
• Keeps the default scheme as GitHub-compatible headers.
• Registers Stripe config webhooks with the Stripe signature scheme.
• Adds regression coverage for both the shared dispatcher and Stripe seeding path.

Validation

• pnpm install --frozen-lockfile
• pnpm --filter @emulators/core build
• pnpm --filter @emulators/core test
• pnpm --filter @emulators/stripe test
• pnpm --filter @emulators/core type-check
• pnpm --filter @emulators/stripe type-check
• pnpm --filter @emulators/core lint
• pnpm --filter @emulators/stripe lint

Lint currently reports pre-existing warnings in untouched files:

• packages/@emulators/core/src/fonts.ts
• packages/@emulators/stripe/src/helpers.ts
• packages/@emulators/stripe/src/routes/checkout-sessions.ts

Closes #97.
Closes #98.

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