fix: Replace all remaining hardcoded English reasoning with structured codes

This commit removes the last hardcoded English text from reasoning fields
across all backend services, completing the i18n implementation.

Changes by service:

Safety Stock Calculator (safety_stock_calculator.py):
- CALC:STATISTICAL_Z_SCORE - Statistical calculation with Z-score
- CALC:ADVANCED_VARIABILITY - Advanced formula with demand and lead time variability
- CALC:FIXED_PERCENTAGE - Fixed percentage of lead time demand
- All calculation methods now use structured codes with pipe-separated parameters

Price Forecaster (price_forecaster.py):
- PRICE_FORECAST:DECREASE_EXPECTED - Price expected to decrease
- PRICE_FORECAST:INCREASE_EXPECTED - Price expected to increase
- PRICE_FORECAST:HIGH_VOLATILITY - High price volatility detected
- PRICE_FORECAST:BELOW_AVERAGE - Current price below average (buy opportunity)
- PRICE_FORECAST:STABLE - Price stable, normal schedule
- All forecasts include relevant parameters (change_pct, days, etc.)

Optimization Utils (shared/utils/optimization.py):
- EOQ:BASE - Economic Order Quantity base calculation
- EOQ:MOQ_APPLIED - Minimum order quantity constraint applied
- EOQ:MAX_APPLIED - Maximum order quantity constraint applied
- TIER_PRICING:CURRENT_TIER - Current tier pricing
- TIER_PRICING:UPGRADED - Upgraded to higher tier for savings
- All optimizations include calculation parameters

Format: All codes use pattern "CATEGORY:TYPE|param1=value|param2=value"
This allows frontend to parse and translate with parameters while maintaining
technical accuracy for logging and debugging.

Frontend can now translate ALL reasoning codes across the entire system.

services/procurement/app/services/safety_stock_calculator.py

@@ -117,10 +117,8 @@ class SafetyStockCalculator:
         # Determine confidence
         confidence = self._determine_confidence(demand_std_dev, lead_time_days)
 
-        reasoning = (
-            f"Service level {service_level*100:.1f}% (Z={z_score:.2f}) × "
-            f"Demand σ={demand_std_dev:.2f} × √{lead_time_days} days"
-        )
+        # Use calculation method as reasoning code with parameters
+        reasoning = f"CALC:STATISTICAL_Z_SCORE|service_level={service_level*100:.1f}|z_score={z_score:.2f}|demand_std={demand_std_dev:.2f}|lead_time={lead_time_days}"
 
         return SafetyStockResult(
             safety_stock_quantity=Decimal(str(round(safety_stock, 2))),
@@ -211,10 +209,8 @@ class SafetyStockCalculator:
 
         confidence = 'high' if lead_time_std_dev > 0 else 'medium'
 
-        reasoning = (
-            f"Advanced formula considering both demand variability "
-            f"(σ={demand_std_dev:.2f}) and lead time variability (σ={lead_time_std_dev:.1f} days)"
-        )
+        # Use calculation method as reasoning code with parameters
+        reasoning = f"CALC:ADVANCED_VARIABILITY|demand_std={demand_std_dev:.2f}|lead_time_std={lead_time_std_dev:.1f}"
 
         return SafetyStockResult(
             safety_stock_quantity=Decimal(str(round(safety_stock, 2))),
@@ -249,7 +245,8 @@ class SafetyStockCalculator:
         lead_time_demand = average_demand * lead_time_days
         safety_stock = lead_time_demand * percentage
 
-        reasoning = f"{percentage*100:.0f}% of lead time demand ({lead_time_demand:.2f})"
+        # Use calculation method as reasoning code with parameters
+        reasoning = f"CALC:FIXED_PERCENTAGE|percentage={percentage*100:.0f}|lead_time_demand={lead_time_demand:.2f}"
 
         return SafetyStockResult(
             safety_stock_quantity=Decimal(str(round(safety_stock, 2))),