feat: add developer platform reliability tools

[codeX-james]

Summary

• add stable idempotency keys and retry helpers to the TypeScript SDK
• enable structured request logs with request and trace correlation IDs
• expose a searchable marketplace directory for built-in integrations
• add a redacted, in-memory webhook receiver for local development
• document the new developer platform capabilities

Verification

• 16 focused tests pass across the backend and SDK
• changed backend files pass ESLint with zero errors
• SDK TypeScript build passes after building its workspace dependency
• full backend TypeScript build remains blocked by pre-existing unrelated errors on main

Closes #933
Closes #934
Closes #935
Closes #936

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