Add SpeechController with POST /api/speech/transcribe for audio
transcription and GET /api/speech/health for provider status.
Uses AudioValidationPipe for file upload validation and returns
results in standard { data: T } envelope.
Includes 10 unit tests covering transcribe with options, error
propagation, and all health status combinations.
Fixes#392
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
