Gate relay invites on optional terms acceptance

[tlongwell-block]

Summary

• add an optional deploy-time invite terms gate configured by BUZZ_INVITE_TERMS_URL and BUZZ_INVITE_TERMS_VERSION
• disable and grey the web invite acceptance button until the configured terms are acknowledged
• enforce acceptance at the claim endpoint with a short-lived relay-signed receipt bound to the invite and policy version
• record accepted policy version, joining pubkey, receipt ID, and acceptance timestamps server-side
• preserve the existing invite UI and claim behavior when both variables are unset

Validation

• targeted relay invite receipt/binding tests pass
• DB migration tests pass
• cargo clippy -p buzz-relay -p buzz-db --all-targets -- -D warnings
• web and desktop TypeScript/lint checks pass
• web production build passes
• desktop JS tests pass: 2,885
• pre-commit hooks pass, including mobile analysis
• pre-push desktop, mobile, and desktop Tauri suites pass

Known unrelated validation issues

• full relay suite: 655 passed; one existing mesh demo timeout (504 vs 200); targeted changed tests pass
• full desktop Rust build is blocked by the pre-existing missing bundled binary desktop/src-tauri/binaries/buzz-acp-aarch64-apple-darwin
• pre-push aggregate Rust unit hook selects sqlx 0.9.0, which requires rustc 1.94 while the hook runs rustc 1.89; buzz-db and buzz-push-gateway therefore do not start in that hook. The targeted DB suite and clippy command above pass in the repository's normal environment.