fix: isolate tests on in-memory SQLite to protect production database

Tests were importing the production engine and dropping all tables on
teardown, corrupting topology.db in the Docker volume. Set DATABASE_URL
to sqlite:///:memory: before any import in the test file, and use
StaticPool in database.py when running against :memory: so all
connections share the same in-memory database.

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

backend/tests/test_validation.py

@@ -15,6 +15,9 @@ from sqlalchemy import text
 import sys, os
 sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 
+# Redirect to an in-memory database so tests never touch the production volume.
+os.environ["DATABASE_URL"] = "sqlite:///:memory:"
+
 from database import engine, Base
 from main import app, _migrate_users_must_change_password, _migrate_users_token_version, _migrate_users