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/responses/contractor.go

@@ -26,7 +26,7 @@ type ContractorUserResponse struct {
 // ContractorResponse represents a contractor in the API response
 type ContractorResponse struct {
 	ID            uint                          `json:"id"`
-	ResidenceID   uint                          `json:"residence_id"`
+	ResidenceID   *uint                         `json:"residence_id"`
 	CreatedByID   uint                          `json:"created_by_id"`
 	AddedBy       uint                          `json:"added_by"` // Alias for created_by_id (KMM compatibility)
 	CreatedBy     *ContractorUserResponse       `json:"created_by,omitempty"`
@@ -87,7 +87,7 @@ func NewContractorUserResponse(u *models.User) *ContractorUserResponse {
 func NewContractorResponse(c *models.Contractor) ContractorResponse {
 	resp := ContractorResponse{
 		ID:            c.ID,
-		ResidenceID:   c.ResidenceID,
+		ResidenceID:   c.ResidenceID, // Already a pointer
 		CreatedByID:   c.CreatedByID,
 		AddedBy:       c.CreatedByID, // Alias for KMM compatibility
 		Name:          c.Name,