Create API webhook, endpoints for subscription payments using Stripe

- Add fields to mark users as subscribed to a specific plan and
  subscription renewal date in DB
- Add ability to unsubscribe a user using their email address
- Expose webhook for stripe to callback confirming payment

src/database/models/__init__.py

@@ -14,7 +14,15 @@ class BaseModel(models.Model):
 
 
 class KhojUser(AbstractUser):
+    class SubscriptionType(models.TextChoices):
+        TRIAL = "trial"
+        STANDARD = "standard"
+
     uuid = models.UUIDField(models.UUIDField(default=uuid.uuid4, editable=False))
+    subscription_type = models.CharField(
+        max_length=20, choices=SubscriptionType.choices, default=SubscriptionType.TRIAL
+    )
+    subscription_renewal_date = models.DateTimeField(null=True, default=None)
 
     def save(self, *args, **kwargs):
         if not self.uuid: