feat(enterprise): add SSO/SAML groundwork — model columns and stub service

- Add sso_enabled, sso_provider, sso_config (JSONB) columns to Account model
  (migration included in branding commit 58e3f27f3e8f)
- Create sso_service.py stub with initiate_sso_login, process_sso_callback,
  validate_sso_config — all raise NotImplementedError (Phase 5)
- Add GET /accounts/me/sso endpoint returning enabled status and provider
- Add SSO section in AccountSettingsPage with Enterprise badge and Contact Us link

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

backend/app/services/sso_service.py

@@ -0,0 +1,26 @@
+"""SSO service stub. Full SAML/OIDC implementation planned for Phase 5."""
+from __future__ import annotations
+
+
+async def initiate_sso_login(account_slug: str) -> str:
+    """Return SSO redirect URL for the given account slug.
+
+    Not yet implemented — SSO is an enterprise Phase 5 feature.
+    """
+    raise NotImplementedError("SSO coming soon")
+
+
+async def process_sso_callback(saml_response: str) -> None:
+    """Process an SSO callback (SAML assertion or OIDC token exchange).
+
+    Not yet implemented — SSO is an enterprise Phase 5 feature.
+    """
+    raise NotImplementedError("SSO coming soon")
+
+
+async def validate_sso_config(config: dict) -> bool:
+    """Validate an SSO configuration dict before storing it on the account.
+
+    Not yet implemented — SSO is an enterprise Phase 5 feature.
+    """
+    raise NotImplementedError("SSO coming soon")