feat: wave-3 audio surfaces — stems, sfx, speech enhance, voice clone, translate (ADR-0024)

- audio.StemSeparator/StemSeparationProvider: Demucs zip transport via
  POST /upstream/<id>/v1/stems (Mode two -> two_stems=vocals; model +
  format fields); bounded zip unpack, entry name -> stem, ext -> MIME.
- SFXModel reuses musicgen against the sync /upstream/<id>/v1/sfx route
  (JSON prompt/seconds/steps/cfg_scale/seed -> WAV); musicgen.Request
  gains CFGScale.
- audio.SpeechEnhancer/SpeechEnhancementProvider:
  POST /upstream/<id>/v1/enhance -> WAV (result reuses SpeechResult).
- SpeechRequest.ReferenceAudio/ReferenceMIME (+WithReferenceAudio):
  llamaswap switches to the chatterbox clone route
  POST /upstream/<id>/v1/audio/speech/upload (input + voice_file),
  wav MIME fallback.
- TranscriptionRequest.Translate (+WithTranslate): translate=true form
  field, language=auto forced when no explicit hint (whisper.cpp default
  en would skip translation).
- httptest contract tests (zip unpack, clone-route switch, translate +
  auto-language injection); ADR-0024 (index row deferred — MJ-A backfills
  the ADR index table and parallel edits would conflict).

Co-Authored-By: Claude Fable 5 <[email protected]>

audio/enhance.go

@@ -0,0 +1,66 @@
+package audio
+
+import "context"
+
+// EnhancementRequest asks a speech-enhancement backend (DeepFilterNet style)
+// to denoise a recording. Audio is carried as bytes (never a URL), mirroring
+// TranscriptionRequest (ADR-0024).
+type EnhancementRequest struct {
+	// Audio is the encoded audio to enhance.
+	Audio []byte
+
+	// MIME is the audio MIME type (e.g. "audio/mpeg"); "" = let the backend
+	// sniff it.
+	MIME string
+
+	// Filename is the multipart filename hint some backends key their format
+	// detection on; "" derives one from MIME or falls back to "audio".
+	Filename string
+}
+
+// EnhancementOption mutates an EnhancementRequest before it is sent.
+// Reserved for future request settings (the reference backend takes no
+// parameters).
+type EnhancementOption func(*EnhancementRequest)
+
+// Apply returns a copy of the request with all options applied.
+func (r EnhancementRequest) Apply(opts ...EnhancementOption) EnhancementRequest {
+	for _, opt := range opts {
+		opt(&r)
+	}
+	return r
+}
+
+// SpeechEnhancer denoises speech recordings. The result reuses SpeechResult
+// (audio bytes + MIME + Raw) — enhancement is audio-in/audio-out exactly
+// like synthesis.
+type SpeechEnhancer interface {
+	// Enhance returns the denoised audio.
+	Enhance(ctx context.Context, req EnhancementRequest, opts ...EnhancementOption) (*SpeechResult, error)
+}
+
+// SpeechEnhancerModelOption configures a SpeechEnhancer at construction time.
+// Reserved for future per-model settings.
+type SpeechEnhancerModelOption func(*SpeechEnhancerModelConfig)
+
+// SpeechEnhancerModelConfig carries per-model construction settings.
+type SpeechEnhancerModelConfig struct{}
+
+// ApplySpeechEnhancerModelOptions folds options into a config.
+func ApplySpeechEnhancerModelOptions(opts []SpeechEnhancerModelOption) SpeechEnhancerModelConfig {
+	var cfg SpeechEnhancerModelConfig
+	for _, opt := range opts {
+		opt(&cfg)
+	}
+	return cfg
+}
+
+// SpeechEnhancementProvider mints SpeechEnhancers bound to one backend.
+type SpeechEnhancementProvider interface {
+	// Name is the registry identifier for the provider.
+	Name() string
+
+	// SpeechEnhancerModel returns a SpeechEnhancer bound to the given id
+	// (passed through to the backend verbatim; no catalog validation).
+	SpeechEnhancerModel(id string, opts ...SpeechEnhancerModelOption) (SpeechEnhancer, error)
+}