Add opaque 12-digit publicId for unguessable invoice links.

Public viewer and URLs use publicId instead of sequential primary keys.

Co-authored-by: Cursor <cursoragent@cursor.com>

prisma/schema.prisma

@@ -1242,6 +1242,7 @@ model InvoiceTemplateAccount {
 
 model Invoice {
   id                BigInt             @id @default(autoincrement())
+  publicId          String             @unique(map: "idx_invoices_public_id") @map("public_id") @db.VarChar(32)
   businessId        BigInt             @map("business_id")
   ownerScope        InvoiceOwnerScope  @default(platform) @map("owner_scope")
   issuerBusinessId  BigInt?            @map("issuer_business_id")