feat: add settings cache to reduce database queries in MediaScanner

- Add SettingsCache with TTL-based invalidation (30 seconds)
- Cache scan_poll_interval_seconds and auto_scan_enabled settings
- Reduce database queries from every poll/check to once per TTL period
- Improve error handling with proper fallback values
- Simplify boolean parsing with strings.ToLower for consistency

This optimization reduces database load when checking scan settings,
which occurs frequently during media scanning operations.

internal/services/media_scanner.go

@@ -89,6 +89,7 @@ type MediaScanner struct {
 	scan_mutex       sync.Mutex
 	pollInterval     time.Duration
 	watching         atomic.Bool
+	settingsCache    *SettingsCache
 
 	totalFiles int
 	newItems   int
@@ -106,6 +107,7 @@ func NewMediaScanner(db *database.Queries) *MediaScanner {
 	return &MediaScanner{
 		db:               db,
 		watcher:          watcher,
+		settingsCache:    NewSettingsCache(30 * time.Second),
 		dirtyDirs:        make(map[string]time.Time),
 		fileStability:    make(map[string]*atomic.Bool),
 		pollInterval:     60 * time.Second,
@@ -119,38 +121,52 @@ func NewMediaScanner(db *database.Queries) *MediaScanner {
 }
 
 func (s *MediaScanner) GetPollInterval() time.Duration {
-	if s.db == nil {
-		return 30 * time.Second
+	// Check cache first
+	if cached, ok := s.settingsCache.Get("scan_poll_interval_seconds"); ok {
+		if seconds, err := strconv.Atoi(cached); err == nil {
+			return time.Duration(seconds) * time.Second
+		}
 	}
+
+	// Cache miss - query database
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
 
 	setting, err := s.db.GetSystemSetting(ctx, "scan_poll_interval_seconds")
 	if err != nil || setting == "" {
-		return 30 * time.Second
+		return 60 * time.Second
 	}
+
+	// Store in cache
+	s.settingsCache.Set("scan_poll_interval_seconds", setting)
+
+	// Convert to duration
 	seconds, err := strconv.Atoi(setting)
-	if err != nil || seconds < 1 {
-		return 30 * time.Second
+	if err != nil {
+		return 60 * time.Second
 	}
 	return time.Duration(seconds) * time.Second
 }
 
 func (s *MediaScanner) GetAutoScanEnabled() bool {
-	if s.db == nil {
-		return true // default to enabled
+	// Check cache first
+	if cached, ok := s.settingsCache.Get("auto_scan_enabled"); ok {
+		return strings.ToLower(cached) == "true"
 	}
+
+	// Cache miss - query database
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
+
 	setting, err := s.db.GetSystemSetting(ctx, "auto_scan_enabled")
 	if err != nil || setting == "" {
 		return true
 	}
-	enabled, err := strconv.ParseBool(setting)
-	if err != nil {
-		return true
-	}
-	return enabled
+
+	// Store in cache
+	s.settingsCache.Set("auto_scan_enabled", setting)
+
+	return strings.ToLower(setting) == "true"
 }
 
 func (s *MediaScanner) SetAdminID(adminID pgtype.UUID) {