Fix inverted USE_SANDBOX env var logic

[stefangordon]

Problem

USE_SANDBOX is derived with inverted logic:

USE_SANDBOX = os.getenv("USE_SANDBOX", "true").lower() == "false"

This evaluates to True precisely when the env var is set to false. So a user following the README and setting USE_SANDBOX=false for production actually gets routed to the sandbox endpoint, and their production credentials fail with:

OAuth error: 401 - {"ErrorMessage":"Invalid clientId", ...}

Fix

Compare against "true" so the flag matches its name and the documented behavior:

USE_SANDBOX = os.getenv("USE_SANDBOX", "true").lower() == "true"

• USE_SANDBOX=true → sandbox
• USE_SANDBOX=false → production
• unset → defaults to sandbox (unchanged)

Verified locally: with USE_SANDBOX=false the server now requests a token from PRODUCTION and reaches === SERVER READY ===.

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