fix(tests): repair two pre-existing bugs blocking the backend CI gate

1. backend/app/models/network_diagram.py — `nodes` and `edges` columns
   used `server_default="'[]'"` (a Python string), which SQLAlchemy
   wraps in single quotes when generating DDL, producing
   `JSONB DEFAULT '''[]'''` — invalid JSON. Switch to
   `server_default=text("'[]'::jsonb")` so the literal is passed through
   and the table can actually be created. Surfaced on every CI run as
   `asyncpg.exceptions.InvalidTextRepresentationError: invalid input
   syntax for type json` at fixture setup time, cascading hundreds of
   test errors.

2. backend/tests/conftest.py — drop the deprecated session-scoped
   `event_loop` fixture. Since pytest-asyncio 0.23+, the plugin manages
   the loop itself; redefining it with a session scope but never
   `set_event_loop()`-ing it left the loop dangling, so any test that
   called `asyncio.run()` (e.g. `test_tasks_are_isolated`) closed the
   process loop and broke the next async test in the module —
   `test_require_tenant_context_raises_403_when_no_account` was the
   visible casualty in the CI logs.

Verified locally:
- `pytest tests/test_uploads.py::test_upload_success` — was setup-error
  on `network_diagrams` DDL; now passes.
- `pytest tests/test_tenant_context.py` — was 1 fail / 3 pass; now 4/4.

Both are real bugs, not test infrastructure churn. Pre-existing on
main; not introduced here.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/tests/conftest.py

@@ -32,14 +32,6 @@ TEST_DATABASE_URL = os.environ.get(
 )
 
 
-@pytest.fixture(scope="session")
-def event_loop() -> Generator:
-    """Create an instance of the default event loop for each test case."""
-    loop = asyncio.get_event_loop_policy().new_event_loop()
-    yield loop
-    loop.close()
-
-
 @pytest.fixture
 async def test_db() -> AsyncGenerator[AsyncSession, None]:
     """