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/dto/requests/contractor.go

@@ -2,7 +2,7 @@ package requests
 
 // CreateContractorRequest represents the request to create a contractor
 type CreateContractorRequest struct {
-	ResidenceID   uint     `json:"residence_id" binding:"required"`
+	ResidenceID   *uint    `json:"residence_id"`
 	Name          string   `json:"name" binding:"required,min=1,max=200"`
 	Company       string   `json:"company" binding:"max=200"`
 	Phone         string   `json:"phone" binding:"max=20"`