Fix test URL mismatches to match actual router prefixes

- test_admin.py: /admin/* → /users/* (admin router mounted at /users)
- test_decks.py: /api/v1/decks/* → /decks/* (decks router mounted at /decks)
- No production code changes - test files only

backend/pytest.ini

@@ -0,0 +1,8 @@
+[pytest]
+testpaths = tests
+python_files = test_*.py
+python_classes = Test*
+python_functions = test_*
+asyncio_mode = auto
+addopts = -v --tb=short
+pythonpath = .