chore: give the CrowdSec import refusals stable error codes so the UI can translate them #1568

What: the new CrowdSec import refusal messages (flat archive layout, kept-profile notifications, duplicate notification names) are fixed English strings from the server, like all other server messages. A stable error code per refusal would let the UI translate them in the five locales.
Where: backend/internal/api/handlers/crowdsec_handler.go (the msg* constants), frontend/src/pages/CrowdSecConfig.tsx.
Suggested approach: add an error_code field next to error for these refusals and map it to translation keys, keeping the English text as the fallback.
Severity: low. Found while planning #1559/#1554 (#1562).