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/models/contractor.go

@@ -17,8 +17,8 @@ func (ContractorSpecialty) TableName() string {
 // Contractor represents the task_contractor table
 type Contractor struct {
 	BaseModel
-	ResidenceID   uint                  `gorm:"column:residence_id;index;not null" json:"residence_id"`
-	Residence     Residence             `gorm:"foreignKey:ResidenceID" json:"-"`
+	ResidenceID   *uint                 `gorm:"column:residence_id;index" json:"residence_id"`
+	Residence     *Residence            `gorm:"foreignKey:ResidenceID" json:"-"`
 	CreatedByID   uint                  `gorm:"column:created_by_id;index;not null" json:"created_by_id"`
 	CreatedBy     User                  `gorm:"foreignKey:CreatedByID" json:"created_by,omitempty"`