[wireguard] add unique constrain for keys in pool

uncloud_net/models.py

@@ -84,6 +84,13 @@ class WireGuardVPN(models.Model):
 
     wireguard_public_key = models.CharField(max_length=48, unique=True)
 
+    class Meta:
+        constraints = [
+            models.UniqueConstraint(fields=['vpnpool', 'wireguard_public_key'],
+                                    name='wg_key_unique_per_pool')
+        ]
+
+
     @property
     def network_mask(self):
         return self.vpnpool.subnetwork_mask