feat: add server-side speech-to-text with OpenAI provider

[shatfield4]

Pull Request Type

• ✨ feat (New feature)
• 🐛 fix (Bug fix)
• ♻️ refactor (Code refactoring without changing behavior)
• 💄 style (UI style changes)
• 🔨 chore (Build, CI, maintenance)
• 📝 docs (Documentation updates)

Relevant Issues

resolves #4732
resolves #3885

Description

• Adds support for server-side speech-to-text so users can transcribe through cloud providers
• Lays the foundation for upcoming STT provider PRs (Lemonade, OpenAI Compatible, Deepgram)
• Adds OpenAI as the first server-side STT provider with a dynamic model picker that filters for transcription models
• Same mic button UX as before: auto-stops on silence and respects the auto-submit setting, just transcribed by the chosen provider instead of the browser

Visuals (if applicable)

Additional Information

Developer Validations

• I ran yarn lint from the root of the repo & committed changes
• Relevant documentation has been updated (if applicable)
• I have tested my code functionality
• Docker build succeeds locally