Add Prisma schema and initial migration

[iexwr]

Context

The service imports @prisma/client and references PaymentRequest, Cursor, and Webhook models, but there is no prisma/schema.prisma and no migration. npx prisma migrate dev currently fails.

Acceptance criteria

• Add prisma/schema.prisma with:
  • PaymentRequest { id Int @id @default(autoincrement()), contract String, requestId Int, payee String, asset String, amount String, memo String, status String, createdAt Int, expiresAt Int, lastLedger Int }
  • Cursor { id String @id, ledger Int }
  • Webhook { id String @id @default(uuid()), url String, secret String, createdAt DateTime @default(now()) }
• Provide an initial migration under prisma/migrations/.
• npx prisma generate succeeds.

Technical notes

• Store amounts as String (i128 in contract) to avoid numeric precision loss.
• Use provider = "postgresql" with env("DATABASE_URL").

Testing

• npx prisma validate passes; npx prisma migrate dev --name init applies cleanly on a fresh DB.