feat: Complete backend i18n implementation with error codes and demo data

Demo Seed Scripts:
- Updated seed_demo_purchase_orders.py to use structured reasoning_data
  * Imports create_po_reasoning_low_stock and create_po_reasoning_supplier_contract
  * Generates reasoning_data with product names, stock levels, and consequences
  * Removed deprecated reasoning/consequence TEXT fields
- Updated seed_demo_batches.py to use structured reasoning_data
  * Imports create_batch_reasoning_forecast_demand and create_batch_reasoning_regular_schedule
  * Generates intelligent reasoning based on batch priority and AI assistance
  * Adds reasoning_data to all production batches

Backend Services - Error Code Implementation:
- Updated safety_stock_calculator.py with error codes
  * Replaced "Lead time or demand std dev is zero or negative" with ERROR:LEAD_TIME_INVALID
  * Replaced "Insufficient historical demand data" with ERROR:INSUFFICIENT_DATA
- Updated replenishment_planning_service.py with error codes
  * Replaced "Insufficient data for safety stock calculation" with ERROR:INSUFFICIENT_DATA
  * Frontend can now translate error codes using i18n

Demo data will now display with translatable reasoning in EN/ES/EU languages.
Backend services return error codes that frontend translates for user's language.

services/procurement/scripts/demo/seed_demo_purchase_orders.py

@@ -34,6 +34,13 @@ from app.models.purchase_order import (
     PurchaseOrder, PurchaseOrderItem, PurchaseOrderStatus
 )
 
+# Import reasoning helper functions for i18n support
+sys.path.insert(0, str(Path(__file__).parent.parent.parent.parent))
+from shared.schemas.reasoning_types import (
+    create_po_reasoning_low_stock,
+    create_po_reasoning_supplier_contract
+)
+
 # Configure logging
 logger = structlog.get_logger()
 
@@ -116,31 +123,39 @@ async def create_purchase_order(
 
     # Generate reasoning for JTBD dashboard (if columns exist after migration)
     days_until_delivery = (required_delivery - created_at).days
-    reasoning_text = None
-    reasoning_json = None
-    consequence_text = None
+
+    # Generate structured reasoning_data for i18n support
+    reasoning_data = None
 
     try:
-        # Try to set reasoning fields (will work after migration)
+        # Get product names from items
+        product_names = [item.product_name for item in items if hasattr(item, 'product_name')]
+        if not product_names:
+            product_names = [f"Product {i+1}" for i in range(len(items))]
+
         if status == PurchaseOrderStatus.pending_approval:
-            reasoning_text = f"Low stock detected for {supplier.name} items. Current inventory projected to run out in {days_until_delivery + 2} days."
-            consequence_text = f"Stock-out risk in {days_until_delivery + 2} days if not approved. Production may be impacted."
-            reasoning_json = {
-                "trigger": "low_stock",
-                "urgency_score": 75 if days_until_delivery < 5 else 50,
-                "days_remaining": days_until_delivery + 2,
-                "supplier_trust_score": supplier.trust_score
-            }
+            # Low stock detection reasoning
+            days_until_stockout = days_until_delivery + 2
+            reasoning_data = create_po_reasoning_low_stock(
+                supplier_name=supplier.name,
+                product_names=product_names,
+                current_stock=random.uniform(20, 50),  # Demo: low stock
+                required_stock=random.uniform(100, 200),  # Demo: needed stock
+                days_until_stockout=days_until_stockout,
+                threshold_percentage=20,
+                affected_products=product_names[:2] if len(product_names) > 1 else product_names,
+                estimated_lost_orders=random.randint(5, 15) if days_until_stockout <= 3 else None
+            )
         elif auto_approved:
-            reasoning_text = f"Auto-approved based on supplier trust score ({supplier.trust_score:.0%}) and amount within threshold (€{subtotal:.2f})."
-            reasoning_json = {
-                "trigger": "auto_approval",
-                "trust_score": supplier.trust_score,
-                "amount": float(subtotal),
-                "threshold": 500.0
-            }
-    except Exception:
-        # Columns don't exist yet, that's ok
+            # Supplier contract/auto-approval reasoning
+            reasoning_data = create_po_reasoning_supplier_contract(
+                supplier_name=supplier.name,
+                product_names=product_names,
+                contract_terms=f"Auto-approval threshold: €500",
+                trust_score=float(supplier.trust_score) if hasattr(supplier, 'trust_score') else 0.85
+            )
+    except Exception as e:
+        logger.warning(f"Failed to generate reasoning_data: {e}")
         pass
 
     # Create PO
@@ -165,12 +180,10 @@ async def create_purchase_order(
         updated_by=SYSTEM_USER_ID
     )
 
-    # Set reasoning fields if they exist (after migration)
-    if reasoning_text:
+    # Set structured reasoning_data for i18n support
+    if reasoning_data:
         try:
-            po.reasoning = reasoning_text
-            po.consequence = consequence_text
-            po.reasoning_data = reasoning_json
+            po.reasoning_data = reasoning_data
         except Exception:
             pass  # Columns don't exist yet