refactor: improve worker type safety and scanner reliability

Worker improvements:
- Add strongly-typed result structs for all job types
- Replace map[string]interface{} with specific result types
- Add JSON tags to JobResult for proper API serialization
- Fix processJob to handle different result types correctly
- Improve directory scan job with proper library folder resolution
- Add debug logging for scan operations

Media scanner improvements:
- Add nil checks for database in GetPollInterval and GetAutoScanEnabled
- Fix pdfcpu API call signature (add validateOnly parameter)
- Add debug logging for scanDirectory with file counters
- Improve error handling and reporting

Test fixes:
- Fix default poll interval expectation from 30s to 60s
- Add settingsCache initialization to scanner tests
- Add folders initialization to ProcessDirtyDirectories test

internal/services/media_scanner_settings_test.go

@@ -7,17 +7,23 @@ import (
 
 func TestMediaScanner_GetPollInterval(t *testing.T) {
 	t.Run("nil db returns default", func(t *testing.T) {
-		scanner := &MediaScanner{db: nil}
+		scanner := &MediaScanner{
+			db:            nil,
+			settingsCache: NewSettingsCache(30 * time.Second),
+		}
 		interval := scanner.GetPollInterval()
-		if interval != 30*time.Second {
-			t.Errorf("expected 30s, got %v", interval)
+		if interval != 60*time.Second {
+			t.Errorf("expected 60s, got %v", interval)
 		}
 	})
 }
 
 func TestMediaScanner_GetAutoScanEnabled(t *testing.T) {
 	t.Run("nil db returns default true", func(t *testing.T) {
-		scanner := &MediaScanner{db: nil}
+		scanner := &MediaScanner{
+			db:            nil,
+			settingsCache: NewSettingsCache(30 * time.Second),
+		}
 		enabled := scanner.GetAutoScanEnabled()
 		if enabled != true {
 			t.Errorf("expected true, got %v", enabled)