Add album art search, auto-match and a whole-album tag editor

Search iTunes, Deezer, MusicBrainz, DuckDuckGo and Google for square covers,
embed them and/or save cover.jpg, auto-match covers for albums, and edit an
album's shared tags and track list in one place.

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

internal/art/prepared.go

@@ -0,0 +1,121 @@
+package art
+
+import (
+	"context"
+	"crypto/rand"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"image"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+	"strconv"
+	"time"
+)
+
+// Prepared images are downloaded or uploaded covers converted to a clean
+// JPEG, waiting to be applied. They live in a cache folder under a random
+// token and expire after a few hours.
+
+type Prepared struct {
+	Token  string `json:"token"`
+	Width  int    `json:"width"`
+	Height int    `json:"height"`
+	Size   int64  `json:"size"`
+	Square bool   `json:"square"`
+}
+
+type Store struct {
+	Dir    string
+	FFmpeg string
+}
+
+var tokenRe = regexp.MustCompile(`^[0-9a-f]{32}$`)
+
+var ErrBadToken = errors.New("unknown or expired image; choose it again")
+
+const preparedTTL = 3 * time.Hour
+
+// Prepare converts image bytes to a JPEG no larger than maxSize pixels on
+// its longest side (0 keeps the original size).
+func (s *Store) Prepare(ctx context.Context, data []byte, maxSize int) (Prepared, error) {
+	if s.FFmpeg == "" {
+		return Prepared{}, errors.New("preparing images needs ffmpeg")
+	}
+	if err := os.MkdirAll(s.Dir, 0o750); err != nil {
+		return Prepared{}, err
+	}
+	s.cleanup()
+	var b [16]byte
+	if _, err := rand.Read(b[:]); err != nil {
+		return Prepared{}, err
+	}
+	token := hex.EncodeToString(b[:])
+	raw := filepath.Join(s.Dir, token+".src")
+	out := filepath.Join(s.Dir, token+".jpg")
+	defer os.Remove(raw)
+	if err := os.WriteFile(raw, data, 0o640); err != nil {
+		return Prepared{}, err
+	}
+	scale := "scale=iw:ih"
+	if maxSize > 0 {
+		m := strconv.Itoa(maxSize)
+		scale = "scale='if(gt(iw,ih),min(" + m + ",iw),-2)':'if(gt(iw,ih),-2,min(" + m + ",ih))'"
+	}
+	ctx, cancel := context.WithTimeout(ctx, 60*time.Second)
+	defer cancel()
+	cmd := exec.CommandContext(ctx, s.FFmpeg, "-v", "error", "-nostdin", "-y", "-i", raw, "-frames:v", "1",
+		"-vf", scale+",format=yuvj420p", "-q:v", "2", "-f", "image2", out)
+	if msg, err := cmd.CombinedOutput(); err != nil {
+		_ = os.Remove(out)
+		return Prepared{}, fmt.Errorf("that file is not a usable image: %s", firstLine(string(msg)))
+	}
+	f, err := os.Open(out)
+	if err != nil {
+		return Prepared{}, err
+	}
+	cfg, _, err := image.DecodeConfig(f)
+	f.Close()
+	if err != nil {
+		_ = os.Remove(out)
+		return Prepared{}, fmt.Errorf("converted image unreadable: %w", err)
+	}
+	fi, _ := os.Stat(out)
+	return Prepared{Token: token, Width: cfg.Width, Height: cfg.Height, Size: fi.Size(), Square: Square(cfg.Width, cfg.Height)}, nil
+}
+
+// Path returns the prepared JPEG for a token.
+func (s *Store) Path(token string) (string, error) {
+	if !tokenRe.MatchString(token) {
+		return "", ErrBadToken
+	}
+	p := filepath.Join(s.Dir, token+".jpg")
+	fi, err := os.Stat(p)
+	if err != nil || time.Since(fi.ModTime()) > preparedTTL {
+		return "", ErrBadToken
+	}
+	return p, nil
+}
+
+func (s *Store) cleanup() {
+	ents, _ := os.ReadDir(s.Dir)
+	for _, en := range ents {
+		if fi, err := en.Info(); err == nil && time.Since(fi.ModTime()) > preparedTTL {
+			_ = os.Remove(filepath.Join(s.Dir, en.Name()))
+		}
+	}
+}
+
+func firstLine(s string) string {
+	for i, r := range s {
+		if r == '\n' {
+			return s[:i]
+		}
+	}
+	if len(s) > 200 {
+		return s[:200]
+	}
+	return s
+}