- Add Balance float64 field to Subscriber struct with JSON and GORM tags
- Set default value to 0 in database schema
- Align PlanID and Status field formatting
- 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
- Remove omitempty from Plan field in Subscriber model
- Update NotifySubscriberUpdate to use any instead of interface{}
- Update sendDeactivationRequest parameter type from interface{} to any
- Reformat struct field alignment for improved readability
- Add SubscriberStatusInactive to SubscriberStatus enum
- Add blank lines after type declarations for consistency
- Adjust whitespace and alignment across all model structs