feat(frontier): add source field to SearchOrganizationTokensResponse.OrganizationToken (#484)

Expose the source field (e.g. system.starter, system.buy, system.awarded,
system.revert, system.overdraft) already stored on billing_transactions.
Lets the frontend render friendly event labels and filter by event type.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: paanSinghCoder

raystack/frontier/v1beta1/frontier.proto

@@ -584,6 +584,7 @@ message SearchOrganizationTokensResponse {
     string user_avatar = 6;
     google.protobuf.Timestamp created_at = 7;
     string org_id = 8;
+    string source = 9;
   }
 
   repeated OrganizationToken organization_tokens = 1;