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>

migrations/003_contractor_optional_residence.up.sql

@@ -0,0 +1,4 @@
+-- Make residence_id optional for contractors
+-- Allows contractors to be personal (no residence) or shared (with residence)
+
+ALTER TABLE task_contractor ALTER COLUMN residence_id DROP NOT NULL;