Add status-based discovery for canceled subscriptions

[nickchang91]

The previous implementation only captured subscription updates through:

1. created_at searches (new subscriptions)
2. Transaction lookahead (renewals/payments)

This missed canceled subscriptions because cancellations don't generate transactions. They only update the subscription's status.

Solution:

Added an approach to capture all subscription updates:

Strategy	What it captures
Created At Search	New subscriptions
Transaction Lookahead	Renewals, payments
Status-Based Discovery	Canceled, Expired, Past Due
Next Billing Date Search	Upcoming renewals

Changes

• Added _fetch_subscriptions_by_status() to search by subscription status
• Added _fetch_subscriptions_by_ids() helper for cleaner code
• Searches Canceled, Expired, and PastDue statuses directly from Braintree API
• Added 7-day lookahead for upcoming billing dates