Add transcription API to v2 module

Migrate speech-to-text transcription types and OpenAI transcriber
implementation from v1. Types are defined in provider/ to avoid
import cycles and re-exported via type aliases from the root package.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

v2/transcriber.go

@@ -0,0 +1,100 @@
+package llm
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+
+	"gitea.stevedudenhoeffer.com/steve/go-llm/v2/provider"
+)
+
+// Transcriber abstracts a speech-to-text model implementation.
+type Transcriber = provider.Transcriber
+
+// TranscriptionResponseFormat controls the output format requested from a transcriber.
+type TranscriptionResponseFormat = provider.TranscriptionResponseFormat
+
+const (
+	TranscriptionResponseFormatJSON        = provider.TranscriptionResponseFormatJSON
+	TranscriptionResponseFormatVerboseJSON = provider.TranscriptionResponseFormatVerboseJSON
+	TranscriptionResponseFormatText        = provider.TranscriptionResponseFormatText
+	TranscriptionResponseFormatSRT         = provider.TranscriptionResponseFormatSRT
+	TranscriptionResponseFormatVTT         = provider.TranscriptionResponseFormatVTT
+)
+
+// TranscriptionTimestampGranularity defines the requested timestamp detail.
+type TranscriptionTimestampGranularity = provider.TranscriptionTimestampGranularity
+
+const (
+	TranscriptionTimestampGranularityWord    = provider.TranscriptionTimestampGranularityWord
+	TranscriptionTimestampGranularitySegment = provider.TranscriptionTimestampGranularitySegment
+)
+
+// TranscriptionOptions configures transcription behavior.
+type TranscriptionOptions = provider.TranscriptionOptions
+
+// Transcription captures a normalized transcription result.
+type Transcription = provider.Transcription
+
+// TranscriptionSegment provides a coarse time-sliced transcription segment.
+type TranscriptionSegment = provider.TranscriptionSegment
+
+// TranscriptionWord provides a word-level timestamp.
+type TranscriptionWord = provider.TranscriptionWord
+
+// TranscriptionTokenLogprob captures token-level log probability details.
+type TranscriptionTokenLogprob = provider.TranscriptionTokenLogprob
+
+// TranscriptionUsage captures token or duration usage details.
+type TranscriptionUsage = provider.TranscriptionUsage
+
+// TranscribeFile converts an audio file to WAV (via ffmpeg) and transcribes it.
+func TranscribeFile(ctx context.Context, filename string, transcriber Transcriber, opts TranscriptionOptions) (Transcription, error) {
+	if transcriber == nil {
+		return Transcription{}, fmt.Errorf("transcriber is nil")
+	}
+
+	wav, err := audioFileToWav(ctx, filename)
+	if err != nil {
+		return Transcription{}, err
+	}
+
+	return transcriber.Transcribe(ctx, wav, opts)
+}
+
+func audioFileToWav(ctx context.Context, filename string) ([]byte, error) {
+	if filename == "" {
+		return nil, fmt.Errorf("filename is empty")
+	}
+
+	if strings.EqualFold(filepath.Ext(filename), ".wav") {
+		data, err := os.ReadFile(filename)
+		if err != nil {
+			return nil, fmt.Errorf("read wav file: %w", err)
+		}
+		return data, nil
+	}
+
+	tempFile, err := os.CreateTemp("", "go-llm-audio-*.wav")
+	if err != nil {
+		return nil, fmt.Errorf("create temp wav file: %w", err)
+	}
+	tempPath := tempFile.Name()
+	_ = tempFile.Close()
+	defer os.Remove(tempPath)
+
+	cmd := exec.CommandContext(ctx, "ffmpeg", "-hide_banner", "-loglevel", "error", "-y", "-i", filename, "-vn", "-f", "wav", tempPath)
+	if output, err := cmd.CombinedOutput(); err != nil {
+		return nil, fmt.Errorf("ffmpeg convert failed: %w (output: %s)", err, strings.TrimSpace(string(output)))
+	}
+
+	data, err := os.ReadFile(tempPath)
+	if err != nil {
+		return nil, fmt.Errorf("read converted wav file: %w", err)
+	}
+
+	return data, nil
+}