refactor(cleanup): remove dead GuestUser/GuestInvitation code and fix test imports

Deleted:
- app/models/guest_user.py
- app/models/guest_invitation.py
- app/routes/guest_auth.py (was orphaned, not imported)
- tests/test_guest_auth.py (tested removed guest auth system)

Modified:
- app/models/__init__.py: removed stale GuestUser/GuestInvitation comments
- app/services/entity_permission_service.py: updated guest permission comment
- tests/test_permission_system_live.py: replaced GuestUser with User+UserTenant(role=guest),
  changed principal_type from "guest" to "user", switched guest test from
  /api/v1/guest/login to regular /api/v1/auth/login endpoint

Frontend: no guest components found, nothing to clean up.
Alembic migrations: historical migrations referencing guest_users/guest_invitations
  tables are left intact (they document DB history).

app/services/entity_permission_service.py

@@ -485,8 +485,8 @@ async def get_effective_access(
                 EntityPermission.principal_id == role_id,
             )
         )
-    # Guest permission — only if user_id is actually a guest_user_id
-    # (Guest users have no groups/roles, only direct entity_permissions)
+    # Guest permission — guests are now regular users with role=guest
+    # (Guest users may have no groups/roles, only direct entity_permissions)
     principal_conditions.append(
         and_(
             EntityPermission.principal_type == "guest",