feat(audio): ffprobe adapter implementing uploads.Prober (ADR-033)

Cover art is not video; non-zero exit is the file's fault (ErrUnreadable),
a missing binary / cancelled ctx / bad output is ours. WaitDelay bounds a
cancelled probe whose child holds the pipe open.

Checked against real ffprobe on iguana: JSON shape matches the fixtures and
/etc/hosts exits 1 with 'Invalid data found when processing input'. The unit
tests drive a stub process; ffprobe is not installed on koala.

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

internal/adapters/audio/ffprobe.go

@@ -0,0 +1,95 @@
+// Package audio holds the adapters that inspect and convert user-supplied audio
+// with ffmpeg's tools (ADR-033). They sit behind small ports so the use case is
+// tested with fakes and only this package knows a subprocess is involved.
+package audio
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"os/exec"
+	"strconv"
+	"time"
+
+	"git.d-ma.be/mathias/tapir/internal/uploads"
+)
+
+// FFprobe implements uploads.Prober by running ffprobe on the spooled file.
+type FFprobe struct {
+	// Bin is the ffprobe executable ("ffprobe" resolves via PATH).
+	Bin string
+}
+
+// killGrace bounds how long Probe waits for a cancelled ffprobe's pipes to close.
+const killGrace = 2 * time.Second
+
+// Probe runs ffprobe and classifies the outcome. A non-zero exit means ffprobe
+// looked at the file and could not read it: that is the FILE's fault and returns
+// uploads.ErrUnreadable. A missing binary, a cancelled context or unparsable
+// output is OURS and is returned as an ordinary error, so it is never reported to
+// the user as their file being bad.
+func (f *FFprobe) Probe(ctx context.Context, path string) (uploads.Probe, error) {
+	bin := f.Bin
+	if bin == "" {
+		bin = "ffprobe"
+	}
+	cmd := exec.CommandContext(ctx, bin,
+		"-v", "error", "-print_format", "json", "-show_format", "-show_streams", path)
+	cmd.WaitDelay = killGrace
+	out, err := cmd.Output()
+	if err != nil {
+		if ctxErr := ctx.Err(); ctxErr != nil {
+			return uploads.Probe{}, fmt.Errorf("audio: ffprobe: %w", ctxErr)
+		}
+		var exit *exec.ExitError
+		if errors.As(err, &exit) {
+			return uploads.Probe{}, fmt.Errorf("%w: ffprobe: %s", uploads.ErrUnreadable, exit.Stderr)
+		}
+		return uploads.Probe{}, fmt.Errorf("audio: run ffprobe: %w", err)
+	}
+	p, err := ParseProbe(out)
+	if err != nil {
+		return uploads.Probe{}, fmt.Errorf("audio: ffprobe output: %w", err)
+	}
+	return p, nil
+}
+
+// probeJSON is the subset of `ffprobe -print_format json` output that matters.
+type probeJSON struct {
+	Streams []struct {
+		CodecType   string `json:"codec_type"`
+		Disposition struct {
+			AttachedPic int `json:"attached_pic"`
+		} `json:"disposition"`
+	} `json:"streams"`
+	Format struct {
+		Duration string `json:"duration"`
+	} `json:"format"`
+}
+
+// ParseProbe turns ffprobe's JSON into a uploads.Probe. A video stream that is
+// only attached cover art (an mp3/m4a with an album picture) is not video. A
+// missing or "N/A" duration yields zero, which uploads.Validate rejects as
+// unreadable rather than accepting an empty recording.
+func ParseProbe(data []byte) (uploads.Probe, error) {
+	var j probeJSON
+	if err := json.Unmarshal(data, &j); err != nil {
+		return uploads.Probe{}, fmt.Errorf("decode: %w", err)
+	}
+	var p uploads.Probe
+	for _, s := range j.Streams {
+		switch s.CodecType {
+		case "audio":
+			p.HasAudio = true
+		case "video":
+			if s.Disposition.AttachedPic == 0 {
+				p.HasVideo = true
+			}
+		}
+	}
+	if secs, err := strconv.ParseFloat(j.Format.Duration, 64); err == nil && secs > 0 {
+		p.Duration = time.Duration(secs * float64(time.Second))
+	}
+	return p, nil
+}