feat(errors): enforce safe API error policy

[Jayking40]

Enforce Safe API Error Responses

Summary

This PR centralizes API error serialization so internal exception details are not returned to clients. All terminal Express errors are mapped through the safe error message policy, with request IDs included for support correlation and redacted structured logs retained for diagnostics.

Changes

• Added centralized terminal error handling for unknown errors, application errors, malformed JSON, and validation failures
• Ensured request IDs are available even when JSON parsing fails
• Fixed terminal handler registration order so all API routes are protected by the final error middleware
• Replaced caught internal error responses in job and DLQ flows with policy-managed errors
• Added integration coverage for unknown errors, application errors, validation errors, malformed JSON, and 404 responses
• Documented the standardized safe error response envelope and status/code mappings

Testing

• npx jest --runTestsByPath src/errors/errorMessagePolicy.integration.test.ts src/errors/appError.test.ts src/errors/safeErrors.test.ts --runInBand --silent
• npx eslint src/app.ts src/index.ts src/errors/appError.ts src/errors/errorMessagePolicy.integration.test.ts src/middleware/errorHandlers.ts src/logger.ts

Note: full build/lint is currently blocked by an unrelated pre-existing syntax error in src/webhookDelivery.ts.

Closes #273

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