fix: address code review findings

- Start Web UI before the blocking initial sync so the dashboard is
  reachable during the (rate-limited, potentially multi-minute) first
  sync; fold the immediate sync into startPeriodicSync's overlap guard
  so it can never race a concurrent tick over the shared DB / MB client.
- Make MarkNotificationSent idempotent: INSERT OR IGNORE for same-second
  PK collisions, and explicitly swallow FK violations when a release was
  pruned by a concurrent re-sync. Prevents a single vanished/duplicate
  release from aborting the digest mark-sent loop and re-sending.
- Do not abort NotifyOnce's mark-sent loop on a single failure; log and
  continue so every release in the batch is marked.
- NULL-safe reads: COALESCE(type,''), COALESCE(release_date,'') in the
  external_releases and unnotified readers to match the cache reader.
- Update/extend tests for the new idempotency and startup contracts.

cmd/naviwatcher/main.go

@@ -131,17 +131,14 @@ 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.
-	if err := a.doSync(ctx); err != nil {
-		if ctx.Err() != nil {
-			return nil
-		}
-		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.
+	// Start the Web UI dashboard FIRST, in its own goroutine, so the dashboard
+	// accepts connections immediately. The initial sync below is throttled by
+	// the MusicBrainz 1 req/s limit and can take many minutes on a large
+	// library (worst case: a fresh DB where every artist needs MBID
+	// resolution) — exactly when an operator is most likely watching. Starting
+	// the server first means the dashboard is reachable (serving cached data)
+	// during that window instead of refusing connections. It serves until ctx
+	// is cancelled, then shuts down gracefully.
 	if a.web != nil {
 		go func() {
 			if err := a.web.Start(ctx); err != nil {
@@ -157,7 +154,10 @@ func (a *App) run(ctx context.Context) error {
 	// disabled (sender nil / enabled false), so always calling it is safe.
 	a.startNotifier(ctx)
 
-	// Kick off the periodic sync+scan loop goroutine.
+	// Kick off the periodic sync+scan loop. It runs an immediate first sync
+	// (governed by the same overlap guard as periodic ticks) so the service
+	// produces results without waiting a full interval, without racing a
+	// concurrent tick over the shared DB and rate-limited MusicBrainz client.
 	a.startPeriodicSync(ctx)
 
 	<-ctx.Done()
@@ -237,25 +237,38 @@ func (a *App) startPeriodicSync(ctx context.Context) {
 	var free = make(chan struct{}, 1)
 	free <- struct{}{}
 
+	// launch starts a guarded sync if the slot is free, returning true when a
+	// sync was started and false when one is already in progress. The in-flight
+	// goroutine returns the token when done.
+	launch := func(label string) bool {
+		select {
+		case <-free:
+			go func() {
+				defer func() { free <- struct{}{} }()
+				if err := a.doSync(ctx); err != nil {
+					if ctx.Err() != nil {
+						return
+					}
+					log.Printf("%s sync+scan failed: %v", label, err)
+				}
+			}()
+			return true
+		default:
+			return false
+		}
+	}
+
+	// Immediate first sync (guarded), so the service produces results without
+	// waiting a full interval and without racing the first ticker fire.
+	launch("Initial")
+
 	for {
 		select {
 		case <-ctx.Done():
 			log.Println("Periodic sync stopped.")
 			return
 		case <-ticker.C:
-			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)
-					}
-				}()
-			default:
+			if !launch("Periodic") {
 				// Previous sync still running; skip this tick.
 				log.Println("Skipping periodic sync: previous sync still in progress.")
 			}