Add Deepgram transcription provider

[half-programmer]

Summary

• add a Deepgram pre-recorded transcription provider using POST /v1/listen
• register it behind DEEPGRAM_API_KEY with nova-3 as the default model
• add mocked tests for auth, request params, availability, response parsing, and error handling

Validation

• Not run locally: GitHub clone/network access was failing in this environment, so this PR was prepared through the GitHub API. The tests are mock-based and do not require a real Deepgram key.

[half-programmer]

Hi, quick follow-up on this provider PR. The implementation is intentionally scoped to a mock-tested Deepgram pre-recorded transcription provider and is linked from the Daytona content submission. I am happy to make any changes needed for the provider API shape or registry conventions.