feat(enterprise): add multi-PSA adapter stubs for Autotask and Halo PSA

- Create AutotaskProvider stub with all abstract methods raising NotImplementedError
- Create HaloPSAProvider stub with all abstract methods raising NotImplementedError
- Add PSA provider grid in IntegrationsPage showing ConnectWise (active),
  Autotask (Coming Soon), and Halo PSA (Coming Soon) with appropriate badges

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/psa/halopsa/__init__.py

@@ -0,0 +1,3 @@
+from app.services.psa.halopsa.provider import HaloPSAProvider
+
+__all__ = ["HaloPSAProvider"]