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/api/server.go

@@ -21,6 +21,7 @@ import (
 
 	"github.com/cdnutter/playlink/internal/backup"
 	"github.com/cdnutter/playlink/internal/engine"
+	"github.com/cdnutter/playlink/internal/notify"
 	"github.com/cdnutter/playlink/internal/fsops"
 	"github.com/cdnutter/playlink/internal/store"
 )
@@ -29,6 +30,7 @@ type Config struct {
 	SecureCookies bool
 	Version       string
 	Backups       *backup.Manager
+	Notifier      *notify.Notifier
 	Restart       func() // stops the server so the process restarts (after a restore)
 }
 
@@ -80,6 +82,7 @@ func (s *Server) Handler() http.Handler {
 			r.Get("/browse", s.handleBrowse)
 			if s.cfg.Backups != nil {
 				s.backupRoutes(r)
+				s.notifyRoutes(r)
 			}
 
 			r.Get("/pairs", s.handleListPairs)
@@ -344,6 +347,7 @@ type pairInput struct {
 	QuarantineDir       string   `json:"quarantine_dir"`
 	Extensions          []string `json:"extensions"`
 	AutoLinkIdentical   bool     `json:"auto_link_identical"`
+	AutoCleanupEmpty    bool     `json:"auto_cleanup_empty"`
 	ScanIntervalMinutes int      `json:"scan_interval_minutes"`
 	RetentionDays       int      `json:"retention_days"`
 	SettleMinutes       int      `json:"settle_minutes"`
@@ -353,7 +357,7 @@ type pairInput struct {
 
 func (in pairInput) apply(p *store.Pair) {
 	p.Name, p.MasterRoot, p.SlaveRoot, p.QuarantineDir = in.Name, in.MasterRoot, in.SlaveRoot, in.QuarantineDir
-	p.Extensions, p.AutoLinkIdentical = in.Extensions, in.AutoLinkIdentical
+	p.Extensions, p.AutoLinkIdentical, p.AutoCleanupEmpty = in.Extensions, in.AutoLinkIdentical, in.AutoCleanupEmpty
 	p.ScanIntervalMinutes, p.RetentionDays, p.SettleMinutes = in.ScanIntervalMinutes, in.RetentionDays, in.SettleMinutes
 	p.ApproxMatching, p.ApproxThreshold = in.ApproxMatching, in.ApproxThreshold
 }