feat: expose file attachment stats as item properties (#37)

Add file_count and files_total_size to item API responses, computed
via batch query on item_files table (no migration needed).

- Add BatchGetFileStats() to audit_queries.go (follows BatchCheckBOM pattern)
- Add file stats to ItemResponse, HandleListItems, HandleGetItem, HandleGetItemByUUID
- Add 'Files' column to ItemTable (default visible in vertical mode)
- Add has_files computed field to audit completeness scoring (weight 1 for manufactured)

internal/api/handlers.go

@@ -260,6 +260,8 @@ type ItemResponse struct {
 	LongDescription *string        `json:"long_description,omitempty"`
 	StandardCost    *float64       `json:"standard_cost,omitempty"`
 	ThumbnailKey    *string        `json:"thumbnail_key,omitempty"`
+	FileCount       int            `json:"file_count"`
+	FilesTotalSize  int64          `json:"files_total_size"`
 	Properties      map[string]any `json:"properties,omitempty"`
 }
 
@@ -304,9 +306,20 @@ func (s *Server) HandleListItems(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Batch-fetch file attachment stats
+	ids := make([]string, len(items))
+	for i, item := range items {
+		ids[i] = item.ID
+	}
+	fileStats, _ := s.items.BatchGetFileStats(ctx, ids)
+
 	response := make([]ItemResponse, len(items))
 	for i, item := range items {
 		response[i] = itemToResponse(item)
+		if fs, ok := fileStats[item.ID]; ok {
+			response[i].FileCount = fs.Count
+			response[i].FilesTotalSize = fs.TotalSize
+		}
 	}
 
 	writeJSON(w, http.StatusOK, response)
@@ -482,7 +495,15 @@ func (s *Server) HandleGetItemByUUID(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	writeJSON(w, http.StatusOK, itemToResponse(item))
+	response := itemToResponse(item)
+	if fileStats, err := s.items.BatchGetFileStats(ctx, []string{item.ID}); err == nil {
+		if fs, ok := fileStats[item.ID]; ok {
+			response.FileCount = fs.Count
+			response.FilesTotalSize = fs.TotalSize
+		}
+	}
+
+	writeJSON(w, http.StatusOK, response)
 }
 
 // HandleGetItem retrieves an item by part number.
@@ -504,6 +525,14 @@ func (s *Server) HandleGetItem(w http.ResponseWriter, r *http.Request) {
 
 	response := itemToResponse(item)
 
+	// File attachment stats
+	if fileStats, err := s.items.BatchGetFileStats(ctx, []string{item.ID}); err == nil {
+		if fs, ok := fileStats[item.ID]; ok {
+			response.FileCount = fs.Count
+			response.FilesTotalSize = fs.TotalSize
+		}
+	}
+
 	// Include properties from current revision if requested
 	if r.URL.Query().Get("include") == "properties" {
 		revisions, err := s.items.GetRevisions(ctx, item.ID)