feat(02-03): Subscriber and ServicePlan Prisma models + service layer
- Add SubscriberStatus (ACTIVE/SUSPENDED/CANCELLED) and BillingType (PREPAID/POSTPAID) enums
- Add ServicePlan model with name, speed, monthlyPrice, billingType, soft-delete
- Add Subscriber model with accountNumber, billingDay, status lifecycle, creditBalance
- Add TenantSettings model with autoSuspendDays and prepaidLeadDays
- Migrate: 20260304145633_add_subscriber_models
- Extend prisma-tenant.ts with subscriber, servicePlan, tenantSettings query scoping
- Create service-plan-service.ts: createServicePlan, updateServicePlan, listServicePlans, deactivateServicePlan
- Create subscriber-service.ts: createSubscriber, updateSubscriber, changeSubscriberStatus, searchSubscribers, getSubscriber, generateAccountNumber
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