fix(auth): explain missing API key permissions

[tgolob]

Fixes #80

Summary

• inspect the ElevenLabs error body before classifying HTTP 401 responses as invalid keys
• explain that missing_permissions requires the Conversational AI read (convai_read) permission and link directly to API-key settings
• keep invalid/expired keys, other access denials (including IP restrictions), network failures, and unexpected errors distinct

Test plan

• jest --testPathIgnorePatterns='e2e' --runInBand — 20 suites, 249 tests passed
• tsc --noEmit
• eslint src -c eslint.config.js — 0 errors (11 pre-existing warnings in unrelated tests)

[PaulAsjes]

Thanks for the contribution, but we just released v1 which is a complete rewrite in Rust, making this obsolete.