feat: restore credit limits and harden SMS sending

api/prisma/migrations/20260714190000_restore_negative_credit_limit/migration.sql

@@ -0,0 +1,2 @@
+ALTER TABLE "TenantAccount"
+  ADD COLUMN "creditCents" INTEGER NOT NULL DEFAULT 0;

api/prisma/schema.prisma

@@ -267,6 +267,7 @@ model TenantAccount {
   id           String   @id @default(cuid())
   tenantId     String
   balanceCents Int      @default(0)
+  creditCents  Int      @default(0)
   status       String   @default("active")
   createdAt    DateTime @default(now())
   updatedAt    DateTime @updatedAt