fix: prevent duplicate partnumbers in stock mappings table

Add duplicate detection before batch insert of auto-mappings:
- Query existing partnumbers from lot_partnumbers table
- Build case-insensitive set of existing entries
- Filter out duplicates before CreateInBatches
- Also prevent duplicates within single batch

This ensures partnumber uniqueness as primary key requires.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

internal/handlers/pricing.go

@@ -1007,7 +1007,7 @@ func (h *PricingHandler) ImportStockLog(c *gin.Context) {
 			"deleted":                     result.Deleted,
 			"unmapped":                    result.Unmapped,
 			"conflicts":                   result.Conflicts,
-			"fallback_matches":            result.FallbackMatches,
+			"auto_mapped":                 result.AutoMapped,
 			"parse_errors":                result.ParseErrors,
 			"qty_parse_errors":            result.QtyParseErrors,
 			"ignored":                     result.Ignored,
@@ -1042,7 +1042,7 @@ func (h *PricingHandler) ImportStockLog(c *gin.Context) {
 			"deleted":                     p.Deleted,
 			"unmapped":                    p.Unmapped,
 			"conflicts":                   p.Conflicts,
-			"fallback_matches":            p.FallbackMatches,
+			"auto_mapped":                 p.AutoMapped,
 			"parse_errors":                p.ParseErrors,
 			"qty_parse_errors":            p.QtyParseErrors,
 			"ignored":                     p.Ignored,