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>

apps/api/src/customers/customers.controller.ts

@@ -45,6 +45,12 @@ export class CustomersController {
     return { candidates: await this.numids.emptyCandidates() };
   }
 
+  /** Suggested folder number for a new customer. Above `:id` like the rest. */
+  @Get("folder/next")
+  async nextFolder() {
+    return { folderNumber: await this.customers.nextFolderNumber() };
+  }
+
   @Get()
   list(
     @Query("query") query?: string,