Make contractor residence optional with visibility rules

- Make residence_id nullable in contractor model
- Add created_by_id field to track contractor creator
- Update access control: personal contractors visible only to creator,
  residence contractors visible to all residence users
- Add database migration for schema changes
- Update admin panel DTOs and handlers for optional residence
- Fix test utilities for new model structure

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

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

internal/testutil/testutil.go

@@ -309,7 +309,7 @@ func MockAuthMiddleware(user *models.User) gin.HandlerFunc {
 // CreateTestContractor creates a test contractor
 func CreateTestContractor(t *testing.T, db *gorm.DB, residenceID, createdByID uint, name string) *models.Contractor {
 	contractor := &models.Contractor{
-		ResidenceID: residenceID,
+		ResidenceID: &residenceID,
 		CreatedByID: createdByID,
 		Name:        name,
 		IsActive:    true,