When users migrate from the legacy `providers` config to the new
`model_list` format, voice transcription silently breaks on Telegram,
Discord and Slack channels.
The gateway was reading the Groq API key exclusively from
`cfg.Providers.Groq.APIKey`, which is empty once the key is defined
only inside a `model_list` entry. The transcriber was never initialized,
so voice messages fell back to a plain `[voice]` placeholder.
This fix also scans `model_list` for any entry whose `model` field
starts with `groq/` and uses its `api_key` as a fallback, preserving
full backward compatibility with the legacy `providers.groq` field.
b9a66248d8