brn: Introduce BRN as a class

app/models/__init__.py

@@ -3,6 +3,7 @@ from datetime import datetime
 from typing import Union, List, Optional, Any
 
 from app.alarms import alarms_for
+from app.brm.brn import BRN
 from app.extensions import db
 from app.models.alarms import Alarm
 
@@ -73,7 +74,7 @@ class AbstractResource(db.Model):  # type: ignore
 
     @property
     @abstractmethod
-    def brn(self) -> str:
+    def brn(self) -> BRN:
         raise NotImplementedError()
 
     def deprecate(self, *, reason: str) -> None: