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

[rschu1ze]

I created a Tinybird account but the UI is rather complex and this README covers it only rudimentary.

@fallintoplace Since you seem familiar with Tinybird, maybe I could ask you to add

• the steps to install the Tinybird CLI in section "Creating an account" above,
• the steps to create and retrieve a "Tinybird Token" (l. 35),
• the steps to run the INSERT (l. 24),
• the exact invocation of run.sh (the PR message has an example)