Improve data integrity on the extCustomerId

[dimitur2204]

Which area(s) of Podkrepi.bg are affected? (leave empty if unsure)

Database

Describe the feature you'd like to request

Currently the extCustomerId is assigned different values from different requests. Take a look at #425 (comment).

Describe the solution you'd like

Check out what that field is used for and change all requests to assign the needed value or leave the field as optional

Describe alternatives you've considered

No response