refactor: finalize media cleanup and prepare scan settings plan

- Complete media scanner cleanup (ebook → media terminology)
- Update remaining comments for consistency
- Add comprehensive scan settings migration plan
- Comment updates in book_matching.go and main.go
- Remove COMPLETE_MEDIA_CLEANUP_PLAN.md (completed)
- Add SCAN_SETTINGS_MIGRATION_PLAN.md for future implementation

internal/handlers/book_matching.go

@@ -12,7 +12,7 @@ import (
 	"github.com/labstack/echo/v4"
 )
 
-// Book matching service (added to Handler struct in ebook.go initialization)
+// Book matching service (added to Handler struct in scanner.go initialization)
 func (h *Handler) getMatchingService() *services.BookMatchingService {
 	return services.NewBookMatchingService(h.db)
 }