ci: register browser pytest marker

Silences PytestUnknownMarkWarning from '-m not browser' in GitHub
Actions CI. Any test requiring headed Chromium (Kasada bypass,
scraper integration) should be decorated with @pytest.mark.browser
so it is excluded from CI automatically.

Closes #40

pyproject.toml

@@ -39,6 +39,9 @@ include = ["app*", "api*"]
 
 [tool.pytest.ini_options]
 testpaths = ["tests"]
+markers = [
+    "browser: tests that require a headed Chromium browser (Xvfb + playwright install required)",
+]
 
 [tool.ruff]
 line-length = 100