Fix Tinybird benchmark runner auth

[fallintoplace]

What changed

• Replaced the placeholder auth header with a required TINYBIRD_TOKEN environment variable.
• Switched Tinybird requests to Authorization: Bearer ... and made curl fail on HTTP errors.
• Made the runner fail fast on shell errors and invalid/non-numeric statistics.elapsed values.
• Documented the required token in the Tinybird README.

Why

AUTH_HEADER=<TOKEN> is parsed by the shell as redirection syntax, so tinybird/run.sh fails before it can run any benchmark query.

Validation

• bash -n tinybird/run.sh
• env -u TINYBIRD_TOKEN ./tinybird/run.sh
• mocked successful Tinybird responses and checked the output is a 43-entry JSON array with three numeric timings per query
• mocked a non-numeric statistics.elapsed response and confirmed the runner exits non-zero