feat(server): add warmup file parameter for model initialization

[sachaarbonel]

Summary

Adds a CLI flag --warmup-file to the whisper.cpp server that allows running a transcription on a specified audio file during startup to warm up the model before serving requests.

Changes

• Added warmup_file parameter to whisper_params struct
• Added -wf/--warmup-file CLI flag to specify path to warmup audio file
• Implemented warmup logic that runs a quick transcription during model initialization
• Added help text documentation for the new flag

Implementation Details

• Warmup runs after model initialization but before server starts accepting requests
• Uses optimized parameters for fast warmup (single segment, no context, minimal audio context)
• Provides timing feedback showing how long the warmup took
• Gracefully handles warmup failures with warning messages
• Only executes warmup when --warmup-file flag is provided

Usage

./whisper-server --warmup-file samples/jfk.wav --model models/ggml-base.en.bin

Benefits

• Reduces latency for first transcription request
• Ensures model is fully loaded and optimized before serving
• Useful for production deployments where consistent response times are important

This change is backward compatible and doesn't affect existing functionality when the warmup flag is not used.

[ggerganov]

This warmup logic can be delegated to the script that starts the server process. After it starts the server, simply send a transcription request with a short audio file to do the warmup. This is a better alternative because we won't have to maintain this logic.