fix: set DJANGO_SETTINGS_MODULE for test step in CI

Tests at ../tests/unit import visitran modules which access Django
settings at import time. Without DJANGO_SETTINGS_MODULE set, Django
raises ImproperlyConfigured.

Verified locally: 23 passed, 0 failed

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

Author: abhizipstack

.github/workflows/core-backend-tests.yaml

@@ -39,6 +39,8 @@ jobs:
       # pre-commit checks handled by pre-commit.ci
 
       - name: Run tests
+        env:
+          DJANGO_SETTINGS_MODULE: backend.server.settings.dev
         run: |
           uv run coverage run --rcfile=pyproject.toml --source=. -m \
           pytest -x -vv ../tests/unit \