Notifications, and automatic cleanup of empty folders

Settings → Notifications sends a message to Discord, Pushover, ntfy,
Gotify or a JSON webhook when a scan finds songs that newly need
review, when auto-link replaced identical files, and when an automatic
cleanup ran. Each event can be turned off and each service tested; an
optional Playlink address adds a link back to the right page.

A new per-pair option removes empty slave folders after each scan. Only
folders with no file at all inside are removed (into the quarantine,
recorded in History); folders holding any file wait in Cleanup, folders
changed within the settle time are skipped, and dry run removes nothing.

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

internal/engine/engine.go

@@ -14,6 +14,7 @@ import (
 	"time"
 
 	"github.com/cdnutter/playlink/internal/fsops"
+	"github.com/cdnutter/playlink/internal/notify"
 	"github.com/cdnutter/playlink/internal/probe"
 	"github.com/cdnutter/playlink/internal/store"
 )
@@ -42,6 +43,7 @@ type Engine struct {
 	mu sync.Mutex
 
 	beforeAction func(ctx context.Context, reason string) error
+	notify       func(ctx context.Context, ev notify.Event)
 
 	emptyMu sync.Mutex
 	empty   map[int64]*NoMusicScan // latest scan for folders without music, per pair
@@ -366,6 +368,40 @@ func (e *Engine) SetBeforeAction(f func(ctx context.Context, reason string) erro
 	e.beforeAction = f
 }
 
+// SetNotify installs the hook that tells the user about things that
+// happened without them (see the notify package).
+func (e *Engine) SetNotify(f func(ctx context.Context, ev notify.Event)) {
+	e.notify = f
+}
+
+func (e *Engine) emit(ctx context.Context, l *libs, ev notify.Event) {
+	if e.notify == nil {
+		return
+	}
+	ev.PairID, ev.Pair = l.pair.ID, l.pair.Name
+	e.notify(ctx, ev)
+}
+
+func plural(n int, one, many string) string {
+	if n == 1 {
+		return "1 " + one
+	}
+	return fmt.Sprintf("%d %s", n, many)
+}
+
+func formatBytes(n int64) string {
+	const unit = 1024
+	if n < unit {
+		return fmt.Sprintf("%d B", n)
+	}
+	div, exp := int64(unit), 0
+	for m := n / unit; m >= unit; m /= unit {
+		div *= unit
+		exp++
+	}
+	return fmt.Sprintf("%.1f %cB", float64(n)/float64(div), "KMGTPE"[exp])
+}
+
 // BeforeAction runs the pre-action backup, once per user action (not per
 // file). It does nothing in dry run, where no files change. An error means
 // the action must not run.