audio-knife: startup self-test for GOOGLE_APPLICATION_CREDENTIALS

[pragmatrix]

Why

When audio-knife is deployed without a working GOOGLE_APPLICATION_CREDENTIALS setup, the misconfiguration only surfaces when the google-transcribe service fails at request time, which is hard to attribute during deployment. A startup check surfaces the problem immediately in the logs, without preventing startup of the parts that do work.

Summary

• New audio-knife/src/selftest.rs: at startup, verifies GOOGLE_APPLICATION_CREDENTIALS is set, readable, and valid JSON; logs a warning with context on the first problem found.
• Wired into main.rs before the server binds; failures never block startup.
• Workspace version bumped 3.8.0 → 3.8.1 (behavioral change); new VERSIONING.md documents the rule that every behavioral change bumps at least the patch version, with a release history.

Validation

• cargo fmt -- --check clean
• cargo test -p audio-knife: 5 passed
• cargo check -p audio-knife clean at 3.8.1

AI disclosure

This PR was prepared with GitHub Copilot (GLM-5.3-Flash). The change was implemented and reviewed with AI assistance; the final review found two accepted Low-severity notes (JSON-only validation, no dedicated tests).