fix: improve conftest.py with comments and remove unused imports

- Remove unused imports (os, tempfile)
- Add comment explaining app.models import will fail until Task 2.1

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/tests/conftest.py

@@ -1,6 +1,4 @@
-import os
 import sys
-import tempfile
 import pytest
 from pathlib import Path
 
@@ -9,6 +7,9 @@ sys.path.insert(0, str(Path(__file__).parent.parent))
 
 from sqlalchemy import create_engine
 from sqlalchemy.orm import sessionmaker
+
+# NOTE: This import will fail until models are created in Task 2.1
+# This is expected behavior - the models module doesn't exist yet
 from app.models import Base