feat(transcribe): the gateway sends language=sv, omits it for auto, and refuses anything else (#61)

The request used to hard-code language=sv for every upload, so a recording in another
language was decoded as Swedish. The language now comes from the context; no language in
the context is still sv.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/adapters/transcribe/gateway.go

@@ -71,9 +71,20 @@ func (g *Gateway) Transcribe(ctx context.Context, path string) (domain.Transcrip
 		return domain.Transcription{}, fmt.Errorf("transcribe: stat audio: %w", err)
 	}
 
+	// The language is one request field and a closed set: sv is sent, auto sends nothing so
+	// the engine detects it, and anything else never leaves this process (tapir#61).
+	fields := [][2]string{{"model", g.model}, {"response_format", "verbose_json"}}
+	switch lang := domain.LanguageFrom(ctx); lang {
+	case domain.LangSwedish:
+		fields = append(fields, [2]string{"language", string(lang)})
+	case domain.LangAuto:
+	default:
+		return domain.Transcription{}, fmt.Errorf("transcribe: language %q is not one tapir sends", lang)
+	}
+
 	var prefix bytes.Buffer
 	mw := multipart.NewWriter(&prefix)
-	for _, kv := range [][2]string{{"model", g.model}, {"response_format", "verbose_json"}, {"language", "sv"}} {
+	for _, kv := range fields {
 		if err := mw.WriteField(kv[0], kv[1]); err != nil {
 			return domain.Transcription{}, fmt.Errorf("transcribe: build request: %w", err)
 		}