fix i18n fallback chain and atomic transactions for #382 #394

[balv82]

Summary

• i18n: Fix fallback chain, per-request locale for connectors, UTF-8 plugin loading, docker-compose test seeding
• Certains tickets ne s'annulent pas avec le message "La tâche active de cette demande a changé. Un autre opérateur a probablement effectué une action entre-temps" #382 / Extract interroge parfois FME Flow 3 fois au lieu de 2 fois #394: Fix race conditions and non-atomic transactions causing stuck requests and duplicate FME tasks. Defense-in-depth with optimistic locking (@Version), pessimistic locking (SELECT FOR UPDATE), atomic step calculation (MAX(step)+1), DB unique constraint, and a new RequestTaskService for transactional operations.

Test plan

• 497 integration tests pass (0 failures, 2 pre-existing errors unrelated to changes)
• 6 dedicated concurrency tests in RequestConcurrencyIntegrationTest
• Playwright visual tests: login, dashboard, STANDBY validation, STANDBY cancellation, ERROR request actions, admin pages
• Run sql/update_db.sql on existing databases to add version columns and unique index
• Verify existing requests remain functional after migration (version backfill to 0)