fix: Add missing reasoning field to production timeline items

ProductionTimelineItem schema requires a 'reasoning' field (string), but the
dashboard service was only providing 'reasoning_data'. Added the reasoning
text field with fallback to auto-generated text if not present in batch data.

Fixes Pydantic validation error: 'Field required' for reasoning field.

services/orchestrator/app/services/dashboard_service.py

@@ -525,6 +525,7 @@ class DashboardService:
                 "progress": progress,
                 "readyBy": planned_end.isoformat() if planned_end else None,
                 "priority": batch.get("priority", "MEDIUM"),
+                "reasoning": batch.get("reasoning") or f"Scheduled based on {reasoning_data.get('type', 'forecast demand')}",  # Fallback reasoning text
                 "reasoning_data": reasoning_data  # NEW: Structured data for i18n
             })