Only create subscription object when it does not exist for user

This avoid unnecessarily throwing an internal server error when the
user tries to sign-up using multiple mechanisms (e.g first by email, then
by google oauth)

src/khoj/database/adapters/__init__.py

@@ -224,7 +224,9 @@ async def acreate_user_by_phone_number(phone_number: str) -> KhojUser:
     )
     await user.asave()
 
-    await Subscription.objects.acreate(user=user, type=Subscription.Type.STANDARD)
+    user_subscription = await Subscription.objects.filter(user=user).afirst()
+    if not user_subscription:
+        await Subscription.objects.acreate(user=user, type=Subscription.Type.STANDARD)
 
     return user
 
@@ -296,7 +298,9 @@ async def create_user_by_google_token(token: dict) -> KhojUser:
         user=user,
     )
 
-    await Subscription.objects.acreate(user=user, type=Subscription.Type.STANDARD)
+    user_subscription = await Subscription.objects.filter(user=user).afirst()
+    if not user_subscription:
+        await Subscription.objects.acreate(user=user, type=Subscription.Type.STANDARD)
 
     return user