#524 [API/Race] App installation duplicate on concurrent request

**File:** api/blueprints/v1/tenant_apps.py:174-200 **Problem:** Check for existing installation (SELECT) then INSERT is not atomic. Two concurrent installs can both pass the SELECT check, then one fails with duplicate key error (500) instead of 409 conflict. **Fix:** Catch UniqueViolation: ```python try: cur.execute("INSERT INTO highway.tenant_app_installations ...") except psycopg.errors.UniqueViolation: return error_response("App is already installed", status_code=409, code="APP_ALREADY_INSTALLED") ``` **Impact:** 500 errors on concurrent app installations.