fix: address code review findings

- Fix duplicate Telegram notifications: SyncArtistDiscography no longer wipes
  notifications_sent for the whole artist on every cache-miss re-sync; only
  markers for releases that disappear are pruned (FK-safe via INSERT OR REPLACE
  + rgid NOT IN (...)).
- Cache empty MusicBrainz discographies via a new artist_settings.last_synced
  column (migration 009) so zero-release artists honor the TTL instead of being
  re-fetched every cycle.
- Wire the Web UI server and Telegram notifier scheduler into main.run/NewApp.
- Guard startPeriodicSync against overlapping syncs with a done-channel slot.
- Add server.public_url config; NewServerWithConfig derives reachable links
  and no longer advertises the 0.0.0.0 bind address.
- Web handlers: use scanner.ScanArtist per artist, drop always-false
  releaseIgnored lookup and dead endsWith, thread configured threshold.
- Limit :memory: DB pool to one connection so migrations and queries share the
  same in-memory store.

cmd/naviwatcher/main.go

@@ -14,7 +14,9 @@ import (
 	"naviwatcher/internal/database"
 	"naviwatcher/internal/musicbrainz"
 	"naviwatcher/internal/navidrome"
+	"naviwatcher/internal/notifier"
 	"naviwatcher/internal/scanner"
+	"naviwatcher/internal/web"
 )
 
 // App holds all application dependencies for clean shutdown and testability.
@@ -23,6 +25,8 @@ type App struct {
 	db       *database.DB
 	mbClient *musicbrainz.MusicBrainzClient
 	ndClient *navidrome.NavidromeClient
+	web      *web.Server
+	sender   notifier.Sender
 
 	// syncFn, when non-nil, replaces the real syncAndScan call in
 	// startPeriodicSync so tests can observe the loop without live clients.
@@ -94,11 +98,23 @@ func NewApp(ctx context.Context, cfg *config.Config, dbPath string) (*App, error
 		return nil, fmt.Errorf("failed to initialize navidrome client: %w", err)
 	}
 
+	// Build the Web UI dashboard server (not started until run).
+	webServer := web.NewServerWithConfig(cfg, db)
+
+	// Build the notifier sender. A nil sender is fine when Telegram is disabled;
+	// the scheduler is no-op-safe and the web UI needs no sender.
+	var sender notifier.Sender
+	if cfg.Telegram.Enabled {
+		sender = notifier.NewTelegramSender(cfg.Telegram)
+	}
+
 	return &App{
 		cfg:      cfg,
 		db:       db,
 		mbClient: mbClient,
 		ndClient: ndClient,
+		web:      webServer,
+		sender:   sender,
 	}, nil
 }
 
@@ -107,10 +123,6 @@ func (a *App) Close() {
 	if a.mbClient != nil {
 		a.mbClient.Close()
 	}
-	if a.ndClient != nil {
-		// NavidromeClient holds a stateless subsonic client; nothing to close
-		// beyond releasing idle connections tracked by the MusicBrainz client.
-	}
 	if a.db != nil {
 		if err := a.db.Close(); err != nil {
 			log.Printf("Error closing database: %v", err)
@@ -120,9 +132,7 @@ func (a *App) Close() {
 
 func (a *App) run(ctx context.Context) error {
 	// Run an immediate sync+scan so the service produces results without
-	// waiting a full interval, then kick off the periodic loop goroutine.
-	// Business logic added in later tasks (notifier, web server) will be
-	// wired as additional goroutines below.
+	// waiting a full interval.
 	if err := a.doSync(ctx); err != nil {
 		if ctx.Err() != nil {
 			return nil
@@ -130,6 +140,24 @@ func (a *App) run(ctx context.Context) error {
 		log.Printf("Initial sync+scan failed: %v", err)
 	}
 
+	// Start the Web UI dashboard in its own goroutine; it serves until ctx is
+	// cancelled, then shuts down gracefully.
+	if a.web != nil {
+		go func() {
+			if err := a.web.Start(ctx); err != nil {
+				if ctx.Err() != nil {
+					return
+				}
+				log.Printf("Web UI server stopped with error: %v", err)
+			}
+		}()
+	}
+
+	// Start the Telegram notifier scheduler. It is no-op-safe when Telegram is
+	// disabled (sender nil / enabled false), so always calling it is safe.
+	a.startNotifier(ctx)
+
+	// Kick off the periodic sync+scan loop goroutine.
 	a.startPeriodicSync(ctx)
 
 	<-ctx.Done()
@@ -173,28 +201,62 @@ func (a *App) syncAndScan(ctx context.Context) error {
 	return nil
 }
 
+// startNotifier wires the Telegram digest scheduler. The scheduler is
+// no-op-safe (returns without starting when disabled or sender is nil), so it
+// is always safe to call. The base URL for dashboard links comes from the
+// server's configured public_url.
+func (a *App) startNotifier(ctx context.Context) {
+	if !a.cfg.Telegram.Enabled {
+		return
+	}
+	schedule, err := notifier.NewCronSchedule(a.cfg.Telegram.CronSchedule)
+	if err != nil {
+		log.Printf("Notifier schedule invalid (%q): %v; notifier disabled", a.cfg.Telegram.CronSchedule, err)
+		return
+	}
+	uiBaseURL := a.cfg.Server.PublicURL
+	notifier.StartScheduler(ctx, true, schedule, func(ctx context.Context) error {
+		_, err := notifier.NotifyOnce(ctx, a.db, a.sender, a.cfg.Telegram, uiBaseURL)
+		return err
+	}, nil)
+}
+
 // startPeriodicSync runs syncAndScan on a ticker at cfg.Sync.Interval. It
-// blocks until ctx is cancelled, then returns cleanly. Each tick runs in its
-// own goroutine so a slow sync does not block the ticker; a fresh interval is
-// still scheduled regardless.
+// blocks until ctx is cancelled, then returns cleanly. Each tick spawns a
+// goroutine so a slow sync does not block the ticker, but a new sync is
+// skipped while the previous one is still running (guarded by a done channel)
+// so syncs never overlap and contend for the shared DB and rate-limited
+// MusicBrainz client.
 func (a *App) startPeriodicSync(ctx context.Context) {
 	ticker := time.NewTicker(a.cfg.Sync.Interval)
 	defer ticker.Stop()
 
+	// free is a sentinel channel: nil means a sync is currently in flight.
+	var free = make(chan struct{}, 1)
+	free <- struct{}{}
+
 	for {
 		select {
 		case <-ctx.Done():
 			log.Println("Periodic sync stopped.")
 			return
 		case <-ticker.C:
-			go func() {
-				if err := a.doSync(ctx); err != nil {
-					if ctx.Err() != nil {
-						return
+			select {
+			case <-free:
+				// Slot was free; start a sync and release the slot when done.
+				go func() {
+					defer func() { free <- struct{}{} }()
+					if err := a.doSync(ctx); err != nil {
+						if ctx.Err() != nil {
+							return
+						}
+						log.Printf("Periodic sync+scan failed: %v", err)
 					}
-					log.Printf("Periodic sync+scan failed: %v", err)
-				}
-			}()
+				}()
+			default:
+				// Previous sync still running; skip this tick.
+				log.Println("Skipping periodic sync: previous sync still in progress.")
+			}
 		}
 	}
 }