feat: webhook DLQ filters, SDK sandbox mocks, SDK playground, API version notices

[fortezzalaboratory]

What

Resolves the four Stellar Wave issues #929, #930, #931 and #932 in one PR, with one commit per issue.

Issues Resolved

Closes #929
Closes #930
Closes #931
Closes #932

Changes

#929 - Build webhook event replay console

Adds an eventType filter to the DLQ list service and route, and mounts the existing webhooks-dlq router at /api/v1/webhooks, which was not wired into the app.

#930 - Implement API sandbox environment with mock data

Adds SANDBOX_MOCK_PAYMENTS and a deterministic createMockPaymentResult helper to the SDK sandbox module, exported from the SDK index.

#931 - Add SDK playground with interactive examples

Adds packages/sdk/src/playground.ts with runnable read-only examples (sandbox status, refund policy, merchant splits, verification) and docs/sdk/PLAYGROUND.md.

#932 - Build API changelog and versioning notice system

routes/api-versions.ts imported functions that did not exist in middleware/versioning.ts. This adds an in-memory version registry (register, list, get, deprecate), sends Deprecation, Sunset and Link headers for deprecated versions, and mounts the router at /api/v1/api-versions.

Assumptions

• The issue bodies were generic, so the four items were interpreted as the gaps above. Several capabilities already existed in the codebase, and these changes extend them rather than replace them.

How to test

Not run locally. Please review the diffs for the four areas above.

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