feat(customers): a shelf folder number the office can hand back out

The office files every customer in a numbered paper folder and reuses the
number once an account closes. That number was the legacy NUMid, which is
also an import key and the portal login, so reusing it meant re-pointing an
identity. It is now customers.folderNumber: a plain label nothing joins on.

- Unique only among non-archived customers, checked in CustomersService
  (create, update, restore). Archived customers keep theirs as history.
- GET /customers/folder/next suggests the lowest free number; the new
  customer form pre-fills it.
- Backfilled from the utilities NUMid (dev: 1,171 folders, 0 shared); the
  importer copies it on load. Insurance-only customers start without one.
- Reports, the printed statement and the customer file show "Carpeta"
  instead of NUMid. The list searches by it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/database/prisma/schema.prisma

@@ -89,6 +89,13 @@ model Customer {
   // can sort nameless records last — ordering by `name` alone puts them first,
   // since "(" sorts before every letter.
   nameMissing              Boolean   @default(false)
+  // The number on the customer's paper folder on the office shelf. A label,
+  // not an identity: nothing joins on it. The office hands a number back out
+  // once an account is closed, so it is unique only among customers that are
+  // not archived — enforced by CustomersService, not by an index, because
+  // Prisma cannot model a partial/functional unique and would drop one.
+  // Imported customers start with their utilities NUMid.
+  folderNumber             Int?
   addressLine1             String?
   addressLine2             String?
   city                     String?
@@ -128,6 +135,7 @@ model Customer {
   emailNotificationLogs EmailNotificationLog[]
   accountStatusHistory  AccountStatusHistory[]
 
+  @@index([folderNumber])
   @@map("customers")
 }