Export card transactions and categorize expenses via the Brex API

Steps

1. Generate a user token from the Brex Dashboard under Settings > Developer, or use OAuth2 with authorization server https://accounts-api.brex.com/oauth2/default
2. Pass the token as a Bearer value in the Authorization header on all requests
3. Fetch card transactions with GET /v2/transactions/card/primary, filtering by posted_at_start and posted_at_end
4. Retrieve expense records with GET /v1/expenses/card to access merchant data, receipts, and coding
5. Update expense memo or department coding with PATCH /v1/expenses/card/{expense_id}