fix: address code review findings

- Honor ignore_singles/ignore_compilations at scanner read time so toggles
  take effect immediately on the dashboard, artist page, and digest instead
  of waiting for the MusicBrainz cache to expire and prune rows.
- Run notifier notify synchronously in the scheduler loop to avoid overlapping
  read-send-mark runs double-sending the digest.
- Show artist name (with ID fallback) on the archive page instead of raw IDs.
- Select last_synced in GetAllArtistSettings for contract consistency.
- Fix stale startPeriodicSync comment and remove redundant error var.
- Remove dead ignored-branch from the artist template (never rendered).
- Add tests: CSRF sameOrigin, ArtistCacheFresh, secondary_types round-trip,
  and scanner type-toggle filtering.
- Update Specification.md schema/config to reflect mbid, last_synced,
  secondary_types, sync.interval, and server.public_url.

cmd/naviwatcher/main.go

@@ -231,7 +231,9 @@ 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.
+	// free is a buffered token (capacity 1). A sync is in flight while the token
+	// is drained; the in-flight goroutine returns it when done so the next tick
+	// can start a new sync. While the token is held, ticks are skipped.
 	var free = make(chan struct{}, 1)
 	free <- struct{}{}