feat: correlate requests, paginate audit, sign webhooks

[aetheron06]

Closes #55
Closes #61
Closes #57

• Validate and propagate request IDs through request context, events, and queued job metadata.
• Add bounded cursor pagination and filters to audit history.
• Sign outbound webhook body bytes using versioned, event-bound HMAC-SHA256. Keep endpoint secrets out of queued jobs and disclose them only on create or rotation.

Webhook verification example (use the endpoint secret returned at creation or rotation; retain the raw request body):

import { createHmac, timingSafeEqual } from 'node:crypto';

const input = Buffer.concat([
  Buffer.from(`v1.${timestamp}.${eventId}.`, 'utf8'),
  rawBody,
]);
const expected = createHmac('sha256', secret).update(input).digest();
const match = /^v1=([0-9a-f]{64})$/.exec(signatureHeader);
const actual = match && Buffer.from(match[1], 'hex');
const valid = !!actual &&
  actual.length === expected.length &&
  timingSafeEqual(actual, expected);

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[Cjay-Cyber-2]

CI is failing in build-and-test during TypeScript compilation. Please update the stale test fixtures to the current APIs: SorobanSimulationResult no longer accepts id, error values use the { code, message } shape, remove unused injected service mocks, use the current transaction input (asset plus required metadata) instead of assetCode-only payloads, and replace assertions for the removed StellarService.simulateTransaction method with the current simulation API/response shape. Push the fixes and rerun CI.

[Cjay-Cyber-2]

CI Checks Failed - Action Needed. build-and-test fails on stale TypeScript tests against the current API. Update fixtures and mocks to current contracts: use asset and the current transaction result shape, remove obsolete simulateTransaction and getTransactionInfo references, fix the Redis/Nest test setup, and remove unused variables/imports. Push and rerun CI.