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/task_test.go

@@ -205,8 +205,9 @@ func TestTaskCompletion_JSONSerialization(t *testing.T) {
 }
 
 func TestContractor_JSONSerialization(t *testing.T) {
+	residenceID := uint(1)
 	contractor := Contractor{
-		ResidenceID: 1,
+		ResidenceID: &residenceID,
 		CreatedByID: 1,
 		Name:        "Mike's Plumbing",
 		Company:     "Mike's Plumbing Co.",