Prevent creation of invoices where sender is the same as the recipient

docs/openapi.yaml

@@ -769,6 +769,8 @@ paths:
             application/json:
               schema:
                 $ref: '#/components/schemas/Invoice'
+        400:
+          description: Invalid request.
         404:
           description: Sender or recipient not found.
         418:

src/mastodon_api/subscriptions/views.rs

@@ -177,6 +177,9 @@ async fn create_invoice_view(
         .ok_or(HttpError::NotSupported)?
         .monero_config()
         .ok_or(HttpError::NotSupported)?;
+    if invoice_data.sender_id == invoice_data.recipient_id {
+        return Err(ValidationError("sender must be different from recipient").into());
+    };
     let db_client = &**get_database_client(&db_pool).await?;
     let sender = get_profile_by_id(db_client, &invoice_data.sender_id).await?;
     let recipient = get_user_by_id(db_client, &invoice_data.recipient_id).await?;