Add Apple Sign In welcome email, notification preferences on registration, and contractor sharing tests

- Send welcome email to new users who sign up via Apple Sign In
- Create notification preferences (all enabled) when new accounts are created
- Add comprehensive integration tests for contractor sharing:
  - Personal contractors only visible to creator
  - Residence-tied contractors visible to all users with residence access
  - Update/delete access control for shared contractors

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

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

internal/services/contractor_service.go

@@ -198,6 +198,8 @@ func (s *ContractorService) UpdateContractor(contractorID, userID uint, req *req
 	if req.IsFavorite != nil {
 		contractor.IsFavorite = *req.IsFavorite
 	}
+	// If residence_id is not sent in the request (nil), it means the user
+	// removed the residence association - contractor becomes personal
 	contractor.ResidenceID = req.ResidenceID
 
 	if err := s.contractorRepo.Update(contractor); err != nil {