[P2] Set the public Spotify client authentication method in the configuration example #64

Recorded during review of main at a1eb305ccfa2f3fc8ddede4875765cf1b099bd2b. This issue records a finding only; no implementation changes were made or requested.

Finding

The example and Spotify setup text say no client secret is needed, but omit OAUTH_SPOTIFY_CLIENT_AUTH_METHOD=none. Generic provider registration defaults to client_secret_post, which requires a secret unless the method is explicitly set to none (or fails earlier if incompatible with metadata). The later generic migration note does not make the platform-specific example complete.

Evidence

• .env.example:27
• README.md:114
• src/providers.rs:131

Desired outcome

Include the required public-client method setting in the Spotify example and setup instructions without introducing provider-name special cases in code.