feat: parallelize audio chunk transcription for improved performance

[majiayu000]

Fixes #1842

Changes

• Add goroutines with sync.WaitGroup to process audio chunks in parallel
• Use buffered channel to collect transcription results
• Sort results by index to maintain correct ordering

[ksylvan]

APPROVED ✅

The implementation is correct, follows Go best practices for concurrency, and achieves the primary goal of parallelizing chunk transcription. The code compiles cleanly and all existing tests pass.

@majiayu000 ideas for future enhancement:

1. Consider adding a configurable concurrency limit (--max-concurrent flag) if rate limiting becomes an issue
2. Consider adding unit tests for the transcription functionality