- Payment code: crypt() with an empty salt returns '*0' for any code since
  PHP 8, and password changes left the salt empty since FOSUser removal:
  any code was accepted. Generate a salt again on password change and on
  code definition; PaymentCodeHasher rejects empty salts/hashes and uses
  hash_equals.
- /encaissement: account, payment code and conventionnement maximum checks
  are now also done at the validation step, which could be posted directly.
- tests/Smoke/PaymentTerminalTest: terminal access, success (prestataire,
  caissier), errors (no code, wrong code, insufficient balance, disabled
  account, unsalted code), conventionnement mode.
- Shared functional test helpers moved to SmokeClientTrait.