feat: Implement OpenAI style local API server for audio transcription

[Yorick-Ryu]

Before Submitting This PR

Please confirm you have done the following:

• I have searched existing issues and pull requests (including closed ones) to ensure this isn't a duplicate
• I have read CONTRIBUTING.md

If this is a feature or change that was previously closed/rejected:

• I have explained in the description below why this should be reconsidered
• I have gathered community feedback (link to discussion below)

Human Written Description

I implemented a local STT API that follows the OpenAI Whisper format. Currently, the Whisper model is only accessible within Handy; however, many users want to leverage this functionality for external tasks like subtitle transcription without loading multiple model instances. This change exposes the speech-to-text capability as a standardized service, allowing users to do more with limited system memory.

Related Issues/Discussions

Fixes # None
Discussion: #241

Community Feedback

#241

Testing

Environment:

• Tested on: macOS 26.2 (Apple Silicon M1 Pro)
• Status: Functional on macOS. Need help testing on Windows and Linux platforms to ensure consistent behavior.

Test Cases:

• Features: Tested by calling the API using curl and Demo: convert MP3 to SRT
• On-demand Loading: Verified via curl that calling the /v1/audio/transcriptions endpoint correctly triggers the model loading process in the background.
• Waiting Mechanism: Confirmed the API response waits until the model is fully loaded before processing the transcription, preventing "Model not loaded" errors.
• Verified Limitations: Tested various audio formats and confirmed only MP3 currently works reliably; documented this behavior and added a "welcome PRs" note in LOCAL_API.md to guide future contributors.

Screenshots/Videos (if applicable)

[Yorick-Ryu]

@cjpais Hello, is there any issue with this PR?

[cjpais]

@Yorick-Ryu please be patient, I have not had time to review it yet

Im currently traveling and haven't been able to look at my laptop much. Handy was featured in wired which has brought in a lot of new issues and discussions I respond to every day

[Yorick-Ryu]

I saw this: https://www.wired.com/story/handy-free-speech-to-text-app/
Congratulations!

[samiulazam]

@cjpais any updates?

[cjpais]

Patience is the key :)

[Aaryan-Kapoor]

+1

[FSerg]

+1

[Yorick-Ryu]

@cjpais Conflicts are fixed. Is it a good time to merge?

[cjpais]

@Yorick-Ryu Please patience I will merge when I have time.