minor: relationship key definitions to strings

Using the objects directly was causing type checking issues. Strings are equivalent so no functional change.

src/iam/models.py

@@ -42,7 +42,7 @@ class Permission(Base):
 		),
 	)
 
-	service_rel = relationship("Service", foreign_keys=[service_id])
+	service_rel = relationship("Service", foreign_keys="Permission.service_id")
 
 	@property
 	def service_name(self):