Remove timestamp field from createBill

[OCzarnecki]

Timestamp takes precedence over date, meaning that the specified date was always ignored.

Tests

Before this PR

Specifying the date as a cli argument has no effect.

$ ./cospend --project playground add augtest2 10.0 -d 2026-08-01 -D
[DEBUG] Request body: amount=10.00&date=2026-08-01&payedFor=11&payer=11&repeat=n&timestamp=1788624676&what=augtest2
[DEBUG] Request: POST http://****/ocs/v2.php/apps/cospend/api/v1/projects/playground/bills
[DEBUG] Headers: OCS-APIRequest=true, Accept=application/json, Auth=Basic olaf:***
[DEBUG] Response: 200 200 OK
Added expense: augtest2
  Amount:   10.00
  Paid by:  Olaf
  Paid for: Olaf
  Date:     2026-08-01
$ date -d @1788624676
Sat Sep  5 05:11:16 PM BST 2026

You can see that the parameters contain both a date arg and a timestamp arg, timestamp takes precedence and is what ends up in the cospend db.

After this PR

Not specifying a date has the date default to today's date (

$ ./cospend --project playground add augtest4 10.0 -D
[DEBUG] Request body: amount=10.00&date=2026-09-05&payedFor=11&payer=11&repeat=n&what=augtest4
[DEBUG] Request: POST http://****/ocs/v2.php/apps/cospend/api/v1/projects/playground/bills
[DEBUG] Headers: OCS-APIRequest=true, Accept=application/json, Auth=Basic olaf:***
[DEBUG] Response: 200 200 OK
Added expense: augtest4
  Amount:   10.00
  Paid by:  Olaf
  Paid for: Olaf

Specifying the date propagates correctly:

/cospend --project playground add augtest3 10.0 -d 2026-09-01 -D
[DEBUG] Request body: amount=10.00&date=2026-09-01&payedFor=11&payer=11&repeat=n&what=augtest3
[DEBUG] Request: POST http://****/ocs/v2.php/apps/cospend/api/v1/projects/playground/bills
[DEBUG] Headers: OCS-APIRequest=true, Accept=application/json, Auth=Basic olaf:***
[DEBUG] Response: 200 200 OK
Added expense: augtest3
  Amount:   10.00
  Paid by:  Olaf
  Paid for: Olaf
  Date:     2026-09-01

I've checked that cospend ends up with what I expect.

[OCzarnecki]

Hey @chenasraf , thanks for making this! This is my first PR on this project, so let me know if you'd like to see anything else from me :)