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_test.go

@@ -92,13 +92,14 @@ func TestWaitForFileStability_UnstableFile(t *testing.T) {
 	select {
 	case stable := <-stableChan:
 		assert.True(t, stable)
-	case <-time.After(5 * time.Second):
+	case <-time.After(7 * time.Second):
 		t.Fatal("waitForFileStability timeout")
 	}
 }
 func TestProcessDirtyDirectories_BatchesScans(t *testing.T) {
 	db := setupTestDB(t)
 	scanner := NewMediaScanner(db)
+	scanner.folders = []string{"/test/folder"}
 	ctx, cancel := context.WithTimeout(context.Background(), 20*time.Second)
 	defer cancel()
 	// Mark directory dirty multiple times rapidly