4 issues: #1368, #1366, #1365, #1329

[Otaiki1]

Closes #1368
Closes #1366
Closes #1365
Closes #1329

Closes #1368: Synchronous throws inside async Express handlers hang requests until timeout

Fixed synchronous throws in async Express handlers that caused requests to hang for 30 seconds until timeout.

Changes:

• POST /admin/allowlist/add: Removed redundant validation throws (validation middleware handles 400)
• DELETE /admin/allowlist/remove: Replaced throw new NotFoundError() with inline res.status(404).json() + return
• GET /admin/impersonate/:wallet: Replaced throw new ValidationError() and throw new ForbiddenError() with inline 400/403 responses
• POST /admin/api-keys/register: Replaced throw new ValidationError() and throw new ForbiddenError() with inline 400/403 responses
• PATCH /admin/webhooks/:id: Replaced throw new NotFoundError() with inline 404 response

All fixes use the pattern res.status(XXX).json({...}); return; to properly send error responses without leaving requests hanging in Express 4.

Closes #1366: Schema snapshots drift when new health-check fields are added to /health or /ready

Stopped early by the maintainer, who reviewed the work so far. Includes changes to backend/schema-snapshots/get-_health.json, backend/schema-snapshots/get-_ready.json, backend/src/__tests__/issues711.test.ts, backend/src/apiContractSnapshots.ts, backend/src/swagger.ts, backend/src/__tests__/openApiContractTests.test.ts.

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Opened with PRmaster after human review.

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