Add invite code registration system for beta

Backend:
- Add InviteCode model with single-use codes
- Add invite API endpoints (create, list, revoke, validate)
- Modify registration to require invite code when enabled
- Add REQUIRE_INVITE_CODE config toggle (default: true)
- Add Alembic migration for invite_codes table

Frontend:
- Add invite code field to registration page
- Validate invite code on blur with visual feedback
- Pass invite code to registration API

Admins can generate invite codes via /api/docs (Swagger UI).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -3,5 +3,6 @@ from .team import Team
 from .tree import Tree
 from .session import Session
 from .attachment import Attachment
+from .invite_code import InviteCode
 
-__all__ = ["User", "Team", "Tree", "Session", "Attachment"]
+__all__ = ["User", "Team", "Tree", "Session", "Attachment", "InviteCode"]