feat: add repair timeline read APIs

backend/app/repairs/exceptions.py

@@ -13,6 +13,14 @@ class RepairMinerNotFound(RepairError):
         super().__init__(f"Miner not found: {miner_id}")
 
 
+class RepairNotFound(RepairError):
+    """Raised when the requested repair does not exist."""
+
+    def __init__(self, repair_id: object):
+        self.repair_id = repair_id
+        super().__init__(f"Repair not found: {repair_id}")
+
+
 class RepairConflict(RepairError):
     """Base exception for a conflicting repair state."""