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/engine/engine.go

@@ -13,6 +13,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/cdnutter/playlink/internal/art"
 	"github.com/cdnutter/playlink/internal/fsops"
 	"github.com/cdnutter/playlink/internal/probe"
 	"github.com/cdnutter/playlink/internal/store"
@@ -45,6 +46,8 @@ type Engine struct {
 	tagTools     tags.Tools
 	beforeAction func(ctx context.Context, reason string) error
 	tagCache     tagCache
+	artClient    *art.Client
+	artStore     *art.Store
 
 	plansMu sync.Mutex
 	plans   map[int64]*organizePlan // latest "rename now" preview per pair
@@ -74,6 +77,7 @@ type Job struct {
 	Error      string         `json:"error"`
 	Results    map[string]int `json:"results"`
 	Skipped    []string       `json:"skipped"`
+	Notes      []string       `json:"notes"`
 	StartedAt  int64          `json:"started_at"`
 	FinishedAt int64          `json:"finished_at,omitempty"`
 
@@ -95,6 +99,15 @@ func (j *Job) tick() { j.update(func(j *Job) { j.Done++ }) }
 
 func (j *Job) count(key string) { j.update(func(j *Job) { j.Results[key]++ }) }
 
+// note records an informational line (e.g. which cover was chosen).
+func (j *Job) note(msg string) {
+	j.update(func(j *Job) {
+		if len(j.Notes) < 300 {
+			j.Notes = append(j.Notes, msg)
+		}
+	})
+}
+
 func (j *Job) skip(msg string) {
 	j.update(func(j *Job) {
 		j.Results["skipped"]++
@@ -118,6 +131,7 @@ func (e *Engine) Job(pairID int64) *Job {
 		cp.Results[k] = v
 	}
 	cp.Skipped = append([]string{}, j.Skipped...) // never nil: the UI expects an array
+	cp.Notes = append([]string{}, j.Notes...)
 	return &cp
 }