[tests,dependencies] install and configure coverage reports

The reports will be printed on the terminal and written in xml.

A new just recipe is added as well: `just cov`

closes: #9

requirements/test.txt

@@ -1,4 +1,4 @@
-# SHA1:9bf147356ad56dbe073ef0973141bdfb94631c9b
+# SHA1:5d0aacdba01d54c2715a30c8df5ee5aef977d83c
 #
 # This file is autogenerated by pip-compile-multi
 # To update, run:
@@ -7,10 +7,12 @@
 #
 -r prod.txt
 attrs==25.3.0
+coverage==7.7.0
 hypothesis==6.129.3
 iniconfig==2.0.0
 packaging==24.2
 pluggy==1.5.0
 pytest==8.3.5
+pytest-cov==6.0.0
 pytest-django==4.10.0
 sortedcontainers==2.4.0