Add optional server frontend pipeline

[0xShug0]

Resolves #313
Resolves #392: Adds opt-in frontend-scoped HTTPS support

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
• add https as a frontend capability, not a pre/post module, using the existing vendored cpp-httplib TLS stack
• keep frontend dependencies opt-in; the default server build does not include codec/TLS frontend dependencies
• split frontend docs into an overview plus module-specific docs, including Windows/Linux LAME setup for mp3_encode

High-Level Design

The core server keeps its stable HTTP API shape: WAV/text in and WAV/text out. Optional frontend modules run in-process around that core handler. Pre-processing modules adapt client requests before core routing, and post-processing modules adapt core responses before returning to the client.

https is separate from the pre/post module contract. It is a frontend-owned listener capability that terminates TLS in-process and forwards the request envelope to the same core handler. The existing plain HTTP transport is unchanged.

At configure time, users select only the frontend capabilities/modules they need:

cmake -S . -B build/debug \
  -DCMAKE_BUILD_TYPE=Debug \
  -DAUDIOCPP_BUILD_SERVER_FRONTENDS=ON \
  '-DAUDIOCPP_SERVER_FRONTEND_MODULES=audio_decode;mp3_encode;https'

Docs

• app/server/frontends/README.md: frontend overview, supported frontend table, pipeline, and contracts
• app/server/frontends/docs/audio_decode.md: MP3/FLAC ASR input module
• app/server/frontends/docs/mp3_encode.md: MP3 TTS output module and LAME install/configuration notes
• app/server/frontends/docs/https.md: HTTPS frontend listener capability
• app/server/frontends/docs/adding_modules.md: developer guide for adding new frontend modules

Validation

Case	Request / command shape	Response / behavior
Default build, no frontend	AUDIOCPP_BUILD_SERVER_FRONTENDS=OFF, build audiocpp_server	Build passed
HTTPS flags on non-HTTPS build	--https-cert-file ... --https-key-file ...	Exit 1: HTTPS frontend support is not available in this build; configure with -DAUDIOCPP_BUILD_SERVER_FRONTENDS=ON -DAUDIOCPP_SERVER_FRONTEND_MODULES=https
HTTPS frontend build	AUDIOCPP_BUILD_NATIVE_MODEL_MANAGER=OFF, AUDIOCPP_BUILD_SERVER_FRONTENDS=ON, AUDIOCPP_SERVER_FRONTEND_MODULES=https	Build passed; HTTPS uses frontend-selected audiocpp_cpp_httplib without native model manager
CLI missing key pair	--https-cert-file cert.pem only	Exit 1: --https-cert-file and --https-key-file must be set together
Bad cert/key files	cert/key paths do not exist	Exit 1: could not initialize HTTPS frontend server from certificate/key files
HTTPS /health	GET https://127.0.0.1:18443/health	200 OK, {"status":"ok","backend":"cpu","models":0,"ui":true,"ui_management":false}
HTTPS UI root	GET https://127.0.0.1:18443/	200 OK, Content-Type: text/html; charset=utf-8, embedded UI returned
HTTPS models list	GET https://127.0.0.1:18443/v1/models	200 OK, {"object":"list","data":[]}
Plain HTTP to HTTPS port	GET http://127.0.0.1:18443/health	Fails before handler routing; curl exit 52, Empty reply from server
Config relative cert/key	https_cert_file: "cert.pem", https_key_file: "key.pem" in config directory	GET /health over HTTPS returned 200 OK; config-relative path resolution works
Config model list	Same config with one lazy dummy model	GET /v1/models returned one unloaded model entry
Config missing key pair	https_cert_file without https_key_file	Exit 1: server https_cert_file and https_key_file must be set together
HTTPS oversized request body	max_request_body_bytes: 8, POST body larger than 8 bytes	413 Payload Too Large before core routing
HTTPS-capable binary without TLS flags	GET http://127.0.0.1:18080/health	200 OK, normal HTTP path still works
HTTPS request to plain HTTP port	GET https://127.0.0.1:18080/health	Fails before handler routing; curl exit 28, connection timed out
mp3_encode dependency lookup without LAME	Configure with AUDIOCPP_SERVER_FRONTEND_MODULES=mp3_encode and no LAME install	Fails with cross-platform message naming lame/lame.h, libmp3lame, conda, and AUDIOCPP_LAME_ROOT
HTTPS configure after LAME lookup change	Configure with AUDIOCPP_SERVER_FRONTEND_MODULES=https	Configure passed; unrelated frontend capability not affected
Frontend docs split	Checked doc links and Markdown paths	Links point to module-specific docs

Also checked:

• git diff --check
• no changes to app/server/http.cpp or app/server/http.h
• no server process left running after validation

[keturn]

Confirmed that running with --https-cert-file and --https-key-file works as intended for #392.

That is, once I enabled the right build options. .devops/ and scripts/ need updating if this is to be included in builds by default.

[0xShug0]

if this is to be included in builds by default.

Thanks for testing! The frontend may go into the default build. Check the discussions here #313