fix(api): retry item creation on duplicate part number collision

The sequence counter (sequences_by_name table) can get out of sync
with the items table if items were seeded/imported directly or if a
previous create failed after incrementing the sequence but before
the insert committed. This causes Generate() to return a part number
that already exists, hitting the unique constraint on items.part_number.

Add a retry loop (up to 5 attempts) in HandleCreateItem that detects
PostgreSQL unique violation errors (SQLSTATE 23505) via pgconn.PgError
and retries with the next sequence value. Non-duplicate errors still
fail immediately. If all retries are exhausted, returns 409 Conflict
instead of 500.

internal/api/handlers.go

@@ -3,6 +3,7 @@ package api
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"net/http"
 	"os"
@@ -12,6 +13,7 @@ import (
 
 	"github.com/alexedwards/scs/v2"
 	"github.com/go-chi/chi/v5"
+	"github.com/jackc/pgx/v5/pgconn"
 	"github.com/kindredsystems/silo/internal/auth"
 	"github.com/kindredsystems/silo/internal/config"
 	"github.com/kindredsystems/silo/internal/db"
@@ -358,13 +360,6 @@ func (s *Server) HandleCreateItem(w http.ResponseWriter, r *http.Request) {
 		},
 	}
 
-	partNumber, err := s.partgen.Generate(ctx, input)
-	if err != nil {
-		s.logger.Error().Err(err).Msg("failed to generate part number")
-		writeError(w, http.StatusBadRequest, "generation_failed", err.Error())
-		return
-	}
-
 	// Determine item type from category
 	itemType := "part"
 	if len(req.Category) > 0 {
@@ -376,32 +371,64 @@ func (s *Server) HandleCreateItem(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
-	// Create item
-	item := &db.Item{
-		PartNumber:      partNumber,
-		ItemType:        itemType,
-		Description:     req.Description,
-		SourcingType:    req.SourcingType,
-		SourcingLink:    req.SourcingLink,
-		LongDescription: req.LongDescription,
-		StandardCost:    req.StandardCost,
-	}
-	if user := auth.UserFromContext(ctx); user != nil {
-		item.CreatedBy = &user.Username
-	}
-
 	properties := req.Properties
 	if properties == nil {
 		properties = make(map[string]any)
 	}
 	properties["category"] = req.Category
 
-	if err := s.items.Create(ctx, item, properties); err != nil {
+	// Retry loop: if the generated part number collides with an existing
+	// item (sequence counter out of sync), generate a new one and retry.
+	const maxRetries = 5
+	var item *db.Item
+	for attempt := 0; attempt < maxRetries; attempt++ {
+		partNumber, err := s.partgen.Generate(ctx, input)
+		if err != nil {
+			s.logger.Error().Err(err).Msg("failed to generate part number")
+			writeError(w, http.StatusBadRequest, "generation_failed", err.Error())
+			return
+		}
+
+		item = &db.Item{
+			PartNumber:      partNumber,
+			ItemType:        itemType,
+			Description:     req.Description,
+			SourcingType:    req.SourcingType,
+			SourcingLink:    req.SourcingLink,
+			LongDescription: req.LongDescription,
+			StandardCost:    req.StandardCost,
+		}
+		if user := auth.UserFromContext(ctx); user != nil {
+			item.CreatedBy = &user.Username
+		}
+
+		err = s.items.Create(ctx, item, properties)
+		if err == nil {
+			break // success
+		}
+
+		// Check if this is a duplicate key error — retry with next sequence
+		var pgErr *pgconn.PgError
+		if errors.As(err, &pgErr) && pgErr.Code == "23505" {
+			s.logger.Warn().
+				Str("part_number", partNumber).
+				Int("attempt", attempt+1).
+				Msg("duplicate part number, retrying with next sequence value")
+			continue
+		}
+
+		// Non-duplicate error, fail immediately
 		s.logger.Error().Err(err).Msg("failed to create item")
 		writeError(w, http.StatusInternalServerError, "create_failed", err.Error())
 		return
 	}
 
+	if item == nil || item.ID == "" {
+		s.logger.Error().Int("retries", maxRetries).Msg("exhausted retries for part number generation")
+		writeError(w, http.StatusConflict, "duplicate_part_number", "Could not generate a unique part number after multiple attempts")
+		return
+	}
+
 	// Tag item with projects if provided
 	if len(req.Projects) > 0 {
 		for _, projectCode := range req.Projects {