fix(ci): set asyncio_default_fixture_loop_scope=function

Ensures each test gets its own event loop, preventing cross-test
connection/future leakage in pytest-asyncio.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/pyproject.toml

@@ -49,6 +49,7 @@ packages = ["app"]
 
 [tool.pytest.ini_options]
 asyncio_mode = "auto"
+asyncio_default_fixture_loop_scope = "function"
 testpaths = ["tests"]
 markers = [
     "integration: marks tests as integration tests requiring PostgreSQL (deselect with '-m \"not integration\"')",