encapsulate the max_drift into the Target so it is open to customization

app/models/target.py

@@ -6,3 +6,11 @@ class Target(BaseModel):
     value: float
     result: Optional[float]
     drift: float = 0.0
+
+    @classmethod
+    def max_drift(cls):
+        return 10 # 10% elasticity
+
+    @classmethod
+    def max_drift_increment(cls):
+        return 1 # 1%