New error-codes.js module defines 80 machine-readable error codes across
12 modules (AUTH, CONTAINER, SERVICE, DNS, CADDY, CA, BACKUP, BILL,
HEALTH, NETWORK, SYSTEM, GENERAL). Format: DC-[MODULE]-[NUMBER].
errorResponse() now surfaces extras.code at top level of JSON body for
client-side handling. Existing callers work unchanged — codes are opt-in.
Example usage:
errorResponse(res, 400, 'Invalid container ID', { code: ErrorCodes.CONTAINER.INVALID_ID })
1560 tests pass. Routes will adopt codes incrementally.