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/services/stock_import.go

@@ -32,7 +32,7 @@ type StockImportProgress struct {
 	Deleted            int64                    `json:"deleted,omitempty"`
 	Unmapped           int                      `json:"unmapped,omitempty"`
 	Conflicts          int                      `json:"conflicts,omitempty"`
-	FallbackMatches    int                      `json:"fallback_matches,omitempty"`
+	AutoMapped         int                      `json:"auto_mapped,omitempty"`
 	ParseErrors        int                      `json:"parse_errors,omitempty"`
 	QtyParseErrors     int                      `json:"qty_parse_errors,omitempty"`
 	Ignored            int                      `json:"ignored,omitempty"`
@@ -49,7 +49,7 @@ type StockImportResult struct {
 	Deleted            int64
 	Unmapped           int
 	Conflicts          int
-	FallbackMatches    int
+	AutoMapped         int
 	ParseErrors        int
 	QtyParseErrors     int
 	Ignored            int
@@ -144,14 +144,15 @@ func (s *StockImportService) Import(
 	}
 
 	var (
-		records         []models.StockLog
-		unmapped        int
-		conflicts       int
-		fallbackMatches int
-		parseErrors     int
-		qtyParseErrors  int
-		ignored         int
-		suggestionsByPN = make(map[string]StockMappingSuggestion)
+		records           []models.StockLog
+		unmapped          int
+		conflicts         int
+		autoMapped        int
+		parseErrors       int
+		qtyParseErrors    int
+		ignored           int
+		suggestionsByPN   = make(map[string]StockMappingSuggestion)
+		autoMappingsToAdd = make(map[string]models.LotPartnumber) // key -> mapping
 	)
 	ignoreRules, err := s.loadIgnoreRules()
 	if err != nil {
@@ -174,19 +175,38 @@ func (s *StockImportService) Import(
 		}
 		partnumber := strings.TrimSpace(row.Article)
 		key := normalizeKey(partnumber)
+		description := strings.TrimSpace(row.Description)
+
+		// Check if already mapped
 		mappedLots := partnumberMatcher.MatchLots(partnumber)
 		if len(mappedLots) == 0 {
-			unmapped++
-			suggestionsByPN[key] = upsertSuggestion(suggestionsByPN[key], StockMappingSuggestion{
-				Partnumber:  partnumber,
-				Description: strings.TrimSpace(row.Description),
-				Reason:      "unmapped",
-			})
+			// Try to auto-map based on prefix match
+			if matchedLot := partnumberMatcher.FindPrefixMatch(partnumber); matchedLot != "" {
+				// Collect for batch insert later
+				descPtr := &description
+				if description == "" {
+					descPtr = nil
+				}
+				autoMappingsToAdd[key] = models.LotPartnumber{
+					Partnumber:  partnumber,
+					LotName:     matchedLot,
+					Description: descPtr,
+				}
+				autoMapped++
+			} else {
+				// No prefix match found
+				unmapped++
+				suggestionsByPN[key] = upsertSuggestion(suggestionsByPN[key], StockMappingSuggestion{
+					Partnumber:  partnumber,
+					Description: description,
+					Reason:      "unmapped",
+				})
+			}
 		} else if len(mappedLots) > 1 {
 			conflicts++
 			suggestionsByPN[key] = upsertSuggestion(suggestionsByPN[key], StockMappingSuggestion{
 				Partnumber:  partnumber,
-				Description: strings.TrimSpace(row.Description),
+				Description: description,
 				Reason:      "conflict",
 			})
 		}
@@ -210,6 +230,45 @@ func (s *StockImportService) Import(
 		})
 	}
 
+	// Batch create auto-mappings (only unique partnumbers)
+	if len(autoMappingsToAdd) > 0 {
+		// Get all existing partnumbers to check for duplicates
+		partnumbersToCheck := make([]string, 0, len(autoMappingsToAdd))
+		for _, mapping := range autoMappingsToAdd {
+			partnumbersToCheck = append(partnumbersToCheck, mapping.Partnumber)
+		}
+
+		var existingPartnumbers []string
+		if err := s.db.Model(&models.LotPartnumber{}).
+			Where("partnumber IN ?", partnumbersToCheck).
+			Pluck("partnumber", &existingPartnumbers).Error; err == nil {
+
+			// Build set of existing partnumbers
+			existingSet := make(map[string]bool, len(existingPartnumbers))
+			for _, pn := range existingPartnumbers {
+				existingSet[strings.ToLower(strings.TrimSpace(pn))] = true
+			}
+
+			// Filter out duplicates
+			mappingsToInsert := make([]models.LotPartnumber, 0, len(autoMappingsToAdd))
+			for _, mapping := range autoMappingsToAdd {
+				pnKey := strings.ToLower(strings.TrimSpace(mapping.Partnumber))
+				if !existingSet[pnKey] {
+					mappingsToInsert = append(mappingsToInsert, mapping)
+					existingSet[pnKey] = true // Mark as added to prevent duplicates within batch
+				}
+			}
+
+			// Insert only unique mappings
+			if len(mappingsToInsert) > 0 {
+				if err := s.db.CreateInBatches(mappingsToInsert, 100).Error; err != nil {
+					// Log error but don't fail the whole import
+					// These will show up as unmapped in suggestions
+				}
+			}
+		}
+	}
+
 	suggestions := collectSortedSuggestions(suggestionsByPN, 200)
 
 	if len(records) == 0 {
@@ -223,7 +282,7 @@ func (s *StockImportService) Import(
 		ValidRows:       len(records),
 		Unmapped:        unmapped,
 		Conflicts:       conflicts,
-		FallbackMatches: fallbackMatches,
+		AutoMapped:    autoMapped,
 		ParseErrors:     parseErrors,
 		QtyParseErrors:  qtyParseErrors,
 		Current:         40,
@@ -289,7 +348,7 @@ func (s *StockImportService) Import(
 		Deleted:            deleted,
 		Unmapped:           unmapped,
 		Conflicts:          conflicts,
-		FallbackMatches:    fallbackMatches,
+		AutoMapped:         autoMapped,
 		ParseErrors:        parseErrors,
 		QtyParseErrors:     qtyParseErrors,
 		Ignored:            ignored,
@@ -308,7 +367,7 @@ func (s *StockImportService) Import(
 		Deleted:            result.Deleted,
 		Unmapped:           result.Unmapped,
 		Conflicts:          result.Conflicts,
-		FallbackMatches:    result.FallbackMatches,
+		AutoMapped:    result.AutoMapped,
 		ParseErrors:        result.ParseErrors,
 		QtyParseErrors:     result.QtyParseErrors,
 		Ignored:            result.Ignored,