fix: high-severity security hardening (Phase B permissions audit)

Phase B addresses 7 high-severity gaps from the permissions audit:

- B1: Enforce tree access check on session start via can_access_tree
- B2: Replace all inline permission helpers with centralized permissions.py
- B3: Fix require_engineer_or_admin to check is_team_admin before role
- B4: Add is_active field on User with enforcement in get_current_active_user
- B5: Add admin user management endpoints (list, get, role, team-admin, deactivate, activate)
- B6: Add rate limiting on auth/invite endpoints via slowapi (disabled in DEBUG)
- B7: Implement refresh token rotation with JTI-based revocation and meaningful logout

Also reduces access token TTL from 15 to 5 minutes and updates CLAUDE.md
with SaaS/MSP context for future planning sessions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -9,6 +9,7 @@ from .tag import TreeTag, tree_tag_assignments
 from .folder import UserFolder, user_folder_trees
 from .step_category import StepCategory
 from .step_library import StepLibrary, StepRating, StepUsageLog
+from .refresh_token import RefreshToken
 
 __all__ = [
     "User",
@@ -26,4 +27,5 @@ __all__ = [
     "StepLibrary",
     "StepRating",
     "StepUsageLog",
+    "RefreshToken",
 ]