security: Auth fails open when token owner can't be resolved #57

Problem

In app.py, if app_owner can't be resolved (SDK unavailable, token invalid, network error), the auth check allows all access:

def check_authorization():
    if not app_owner:
        return True, None  # ← fails open

This means if the Databricks SDK fails to resolve the token owner at startup, the app runs with no authentication — anyone with the URL can access the terminal and all coding agents.

Expected behavior

Should fail closed: if the owner can't be determined, deny all access. Better to have a broken app than an open terminal with a PAT.

Fix

• If app_owner is None and we're running on Databricks Apps (not local dev), return False
• Allow the fail-open behavior only for local development (DATABRICKS_APP_PORT not set or similar)

Impact

Any user who can reach the app URL gets full terminal access with the PAT owner's credentials, including:

• Databricks workspace access
• AI Gateway / model serving
• Unity Catalog data access
• Shell execution