Add optional server frontend pipeline

[0xShug0]

Resolves #313

Summary

• add an opt-in in-process server frontend pipeline with pre/post module hooks
• add explicit module contracts for request and response envelope states
• add audio_decode for MP3/FLAC ASR input via miniaudio
• add mp3_encode for response_format=mp3 TTS output via libmp3lame
• document how to add custom frontend modules

Validation

• configured debug build with -DAUDIOCPP_BUILD_SERVER_FRONTENDS=ON -DAUDIOCPP_SERVER_FRONTEND_MODULES=audio_decode;mp3_encode
• built build/debug/bin/audiocpp_server
• validated MP3 ASR input returns expected text
• validated FLAC ASR input returns expected text
• validated TTS response_format=mp3 returns audio/mpeg and probes as MP3
• ran git diff --check HEAD~1 HEAD