misc: Move pytest.ini to backend directory

Moving the `pytest.ini` file simplifies the `pytest` run command, and
also correctly mounts the file in the development Docker container.

.vscode/tasks.json

@@ -23,7 +23,7 @@
     {
       "label": "Execute tests",
       "type": "shell",
-      "command": "cd backend && uv run pytest -vv -c ../pytest.ini",
+      "command": "cd backend && uv run pytest -vv",
       "problemMatcher": []
     },
     {

DEVELOPER_SETUP.md

@@ -185,7 +185,6 @@ cd backend
 uv run pytest [path/file]
 # or run the following command to run all tests
 # the -vv switch increases the verbosity of the output, providing more detailed information during test execution.
-# -c specifies the path to a configuration file for pytest.
-uv run pytest -vv -c ../pytest.ini
+uv run pytest -vv
 ```
 ````