fix: remove clear-text logging of full API key

Agent-Logs-Url: https://github.com/christianlouis/dmarq/sessions/add3a51d-8235-4d29-8020-589bffa8ebb5

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

backend/app/main.py

@@ -241,11 +241,6 @@ def create_app() -> FastAPI:
             "=" * 80,
         )
 
-        # In development, also log the full key for convenience
-        # This should be removed in production or controlled by environment variable
-        if os.getenv("ENVIRONMENT", "development") == "development":
-            logger.info("Development Mode - Full API Key: %s", api_key)
-
         # One-time migration: if IMAP_* env vars are set and no mail sources exist,
         # create an initial MailSource from those settings so existing deployments
         # continue to work without manual reconfiguration.