Add creation time to Transaction model

server/database-docs.md

@@ -60,5 +60,6 @@ type Transaction = {
   stack_id: string
   amount: integer // cents
   details: string
+  created_at: UTC datetime ie: 2021-04-15T00:06:21.852620Z
 }
 ```

server/server/api/models.py

@@ -1,5 +1,6 @@
 from django.db import models
 from django.contrib.auth.models import User
+from django.utils import timezone
 
 import uuid
 
@@ -28,3 +29,4 @@ class Transaction(models.Model):
     id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
     details = models.TextField()
     amount = models.DecimalField(max_digits=12, decimal_places=2)
+    created_at = models.DateTimeField(default=timezone.now)