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

prisma/schema.prisma

@@ -57,6 +57,17 @@ enum PeriodStatus {
   CLOSED
 }
 
+enum SubscriberStatus {
+  ACTIVE
+  SUSPENDED
+  CANCELLED
+}
+
+enum BillingType {
+  PREPAID
+  POSTPAID
+}
+
 // =============================================================================
 // MODELS
 // =============================================================================
@@ -137,6 +148,87 @@ model AccountingPeriod {
   @@index([tenantId])
 }
 
+/// TenantSettings holds per-tenant configuration for billing and auto-suspension.
+/// One record per tenant, created on first access or at tenant provisioning.
+model TenantSettings {
+  id               String   @id @default(uuid())
+  tenantId         String   @unique
+  /// Days overdue before automatic subscriber suspension (default 30)
+  autoSuspendDays  Int      @default(30)
+  /// Days before billing date to generate prepaid invoices (default 7)
+  prepaidLeadDays  Int      @default(7)
+  createdAt        DateTime @default(now())
+  updatedAt        DateTime @updatedAt
+
+  /// RLS-ready index — always present on tenant-scoped models
+  @@index([tenantId])
+}
+
+/// A ServicePlan defines the internet service offering (speed, price, billing type).
+/// Plans are soft-deleted (isActive=false) to preserve subscriber history.
+model ServicePlan {
+  id           String      @id @default(uuid())
+  tenantId     String
+  name         String
+  /// Human-readable speed description (e.g., "50 Mbps", "100/20 Mbps")
+  speed        String
+  /// Monthly charge for this plan (2 decimal places)
+  monthlyPrice Decimal     @db.Decimal(10, 2)
+  billingType  BillingType
+  description  String?
+  /// Soft-delete: inactive plans cannot be assigned to new subscribers
+  isActive     Boolean     @default(true)
+  createdAt    DateTime    @default(now())
+  updatedAt    DateTime    @updatedAt
+
+  subscribers  Subscriber[]
+
+  /// Plan names must be unique within a tenant
+  @@unique([tenantId, name])
+  /// RLS-ready index — always present on tenant-scoped models
+  @@index([tenantId])
+}
+
+/// A Subscriber is a customer of the ISP — the core billing entity.
+/// All invoices, payments, and collections target subscribers.
+model Subscriber {
+  id            String           @id @default(uuid())
+  tenantId      String
+  /// Auto-generated sequential identifier per tenant (e.g., "SUB-0001")
+  accountNumber String
+  firstName     String
+  lastName      String
+  email         String?
+  phone         String?
+  address       String
+  /// Zone for collector routing (Phase 3)
+  zone          String?
+  servicePlanId String
+  servicePlan   ServicePlan      @relation(fields: [servicePlanId], references: [id])
+  status        SubscriberStatus @default(ACTIVE)
+  /// Day of month for invoice generation — derived from signup date, capped at 28
+  billingDay    Int
+  activatedAt   DateTime         @default(now())
+  suspendedAt   DateTime?
+  cancelledAt   DateTime?
+  /// Per-subscriber auto-suspend override (null = use TenantSettings.autoSuspendDays)
+  autoSuspendDays Int?
+  notes         String?
+  /// Overpayment credit balance — always updated atomically with journal entries.
+  /// This is NOT a stored ledger balance; it tracks credits for the FIFO allocation
+  /// system (02-05) and is always updated within the same transaction as the journal entry.
+  creditBalance Decimal          @default(0) @db.Decimal(10, 2)
+  createdAt     DateTime         @default(now())
+  updatedAt     DateTime         @updatedAt
+
+  /// Account numbers must be unique within a tenant
+  @@unique([tenantId, accountNumber])
+  /// RLS-ready index — always present on tenant-scoped models
+  @@index([tenantId])
+  @@index([tenantId, status])
+  @@index([tenantId, servicePlanId])
+}
+
 /// A User belongs to a Tenant (or is a super-admin with no tenant).
 /// Email uniqueness is enforced per-tenant, not globally.
 /// Super-admins have isSuperAdmin=true and tenantId=null.