FEAT-695: Bloquer la désinscription pour les sessions passées

📃 TICKET NOTION 📃

Summary

• Block unsubscriptions: Project-level setting (blockUnsubscriptions) prevents participants from unsubscribing. Sessions that have already begun also block unsubscription. Volunteering sessions beginning within 2 days can be separately blocked via blockVolunteeringUnsubscribeIfBeginsSoon.
• Rename blockSubscriptions ? blockUnsubscriptions: Field renamed across models, controllers, UI, locales, and a MongoDB migration.
• Subscription pipeline refactor: Duplicated blocking logic across getSessionInfoAlert and SubscribeUnsubscribeButton consolidated into a single pure function getSessionSubscriptionState ? the new single source of truth for button variant, disabled state, alert message, and dimming. SubscribeUnsubscribeButton rewritten in TypeScript as a thin renderer. getSessionInfoAlert.js deleted.
• 32-test suite: Full test coverage for getSessionSubscriptionState covering all branching paths (steward, auto-subscribed, team, blocked unsubscribe, blocked volunteering, normal unsubscribe, subscribe, alert priority, dimming, date conflicts).

Commits

• beeb9852 fix: Prevent modification of session and team subscriptions with MODIFY_REGISTRATION endpoint
• 720fc416 refactor: Rename badly named prop blockSubscriptions in project to blockUnsubscriptions, with migration file
• c31ee4e2 feat: implement blocking in UI with tooltip messages and all. Refactored the getSessionMetadata/getSessionInfoAlert data pipeline
• cdd9d462 test: add getSessionSubscriptionState test suite for subscription pipeline refactor
• d32027ef feat: implement getSessionSubscriptionState ? single source of truth for subscription pipeline
• 35a36d5d refactor: rewrite SubscribeUnsubscribeButton in TypeScript, consuming SessionSubscriptionState
• 9dcc1ba6 refactor: update SessionShow and SessionShowSmall to use getSessionSubscriptionState
• 0ce63281 refactor: delete getSessionInfoAlert.js ? absorbed into getSessionSubscriptionState