wip: update documentation

docs/SERVICE_AVAILABILITY_SYSTEM.md

@@ -330,6 +330,35 @@ dump($filtered);
 - **HTMX Updates**: Availability changes trigger form refreshes
 - **Form Handlers**: Service selections processed by specialized field handlers
 
+## Future Enhancements
+
+### Planned: API Validation Stage
+
+**Important Note**: The current availability system uses XML data that may become outdated during the booking creation process. A future enhancement will implement a **validation stage** that checks final service selections against real-time availability via the BusProNet API before final booking confirmation.
+
+#### Planned Implementation
+- **Pre-Submission Validation**: Before final booking submission, validate all selected services against current API availability
+- **Real-Time Check**: Call BusProNet API to get current availability status
+- **Conflict Resolution**: Handle cases where selected services are no longer available
+- **User Feedback**: Provide clear messaging when services become unavailable during booking process
+
+#### Technical Integration Points
+```php
+// Future validation service (planned)
+class BookingValidationService 
+{
+    public function validateServiceAvailability(BookingCreateDto $bookingDto): ValidationResult;
+    public function resolveAvailabilityConflicts(BookingCreateDto $bookingDto): ConflictResolution;
+}
+```
+
+#### Business Logic
+- **Session Availability**: Current system prevents overbooking within single booking session
+- **API Validation**: Future system will prevent overbooking across all booking sessions system-wide
+- **Two-Stage Protection**: Provides both immediate feedback and final validation
+
+This planned enhancement will complement the existing dynamic availability system by adding a final validation layer that ensures booking integrity against the authoritative BusProNet API data.
+
 ---
 
 **Implementation Status**: ✅ **Completed and Tested**  
@@ -337,4 +366,5 @@ dump($filtered);
 **Integration**: Seamless with existing form system  
 **Performance**: Optimized for real-time updates  
 **Testing**: Verified working in development environment  
+**Future Enhancement**: API validation stage planned for booking finalization  
 **Documentation**: Comprehensive with examples and troubleshooting