- Add SubscriberAccount with balance, usage limits (data/voice/SMS), and status tracking
- Add UsageEvent for tracking usage sessions with volume, rate, and cost
- Add Invoice with line items and status (DRAFT, PENDING, PAID, OVERDUE, CANCELLED)
- Add RatingPlan with rate definitions for data/voice/SMS and monthly fees
- Add Alert with types (LOW_BALANCE, HIGH_USAGE, PAYMENT_FAILED, SYSTEM_ERROR) and severity levels
- Add Pay
