feat: implement secure file upload system with JWT authentication

- Add JWT-based secure file access for local storage with 1-hour expiry
- Implement GORM repository methods for attachment CRUD operations
- Add secure file serving endpoint with token validation
- Update storage interface to support user context in URL generation
- Add comprehensive security features including path traversal protection
- Update documentation with security model and configuration examples
- Add utility functions for hex/byte conversion and UUID validation
- Configure secure file permissions (0600) for uploaded files

🤖 Generated with [Claude Code](https://claude.ai/code)

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

core/api/attachment_integration_test.go

@@ -131,10 +131,10 @@ func TestAttachmentIntegration(t *testing.T) {
 	// Set up the model instance for the API handlers
 	bc := &util.StandardBcrypt{}
 	
-	// Use the existing datastore model which has the attachment implementation
+	// Use the GORM model which has the attachment implementation
 	// We need to create it with the database connection
-	datastoreModel := model.NewModel(nil, bc, types.Config{})
-	model.Instance = datastoreModel
+	gormModel := model.NewGormModel(db, bc, types.Config{})
+	model.Instance = gormModel
 
 	t.Run("Database Integration Test", func(t *testing.T) {
 		// Test direct database operations first