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/notify.go

@@ -0,0 +1,47 @@
+package api
+
+import (
+	"net/http"
+
+	"github.com/go-chi/chi/v5"
+
+	"github.com/cdnutter/playlink/internal/notify"
+)
+
+func (s *Server) notifyRoutes(r chi.Router) {
+	r.Get("/notifications", s.handleGetNotify)
+	r.Put("/notifications", s.handlePutNotify)
+	r.Post("/notifications/test", s.handleTestNotify)
+}
+
+func (s *Server) handleGetNotify(w http.ResponseWriter, r *http.Request) {
+	writeJSON(w, 200, s.cfg.Notifier.Config(r.Context()))
+}
+
+func (s *Server) handlePutNotify(w http.ResponseWriter, r *http.Request) {
+	var c notify.Config
+	if !readJSON(w, r, &c) {
+		return
+	}
+	if err := s.cfg.Notifier.SetConfig(r.Context(), c); err != nil {
+		writeErr(w, 400, err)
+		return
+	}
+	writeJSON(w, 200, s.cfg.Notifier.Config(r.Context()))
+}
+
+// handleTestNotify sends a test message to a target as entered, saved or not.
+func (s *Server) handleTestNotify(w http.ResponseWriter, r *http.Request) {
+	var body struct {
+		Target  notify.Target `json:"target"`
+		BaseURL string        `json:"base_url"`
+	}
+	if !readJSON(w, r, &body) {
+		return
+	}
+	if err := s.cfg.Notifier.Test(r.Context(), body.Target, body.BaseURL); err != nil {
+		writeErr(w, 400, err)
+		return
+	}
+	writeJSON(w, 200, map[string]bool{"ok": true})
+}