practice: preserve attempt history when questions change #72

Related to #22 and the question-versioning step in docs/product-roadmap.md.

Problem

Saved attempts contain question IDs and selected option IDs. The frontend grades them against the current question bank. Changing an answer key can change a historical result; removing an ID can cause an attempt to be discarded during validation.

Scope

• Record the question-revision and compatibility strategy in an ADR, following ADR 0001.
• Keep reviewed definitions in the canonical shared question data. Identify the exact revision used by an attempt; do not accept browser-supplied answer keys or scores.
• Keep retired revisions available for historical review while excluding them from new sessions.
• Define a compatible interpretation or migration for existing unversioned browser and backend records.
• Update TypeScript validation, backend validation, persistence and import/export contracts together. Document rollout and rollback.
• Preserve the 100-attempt limit, payload bounds, account isolation and generation-based reset behavior.
• Keep this separate from the planned game-history contract in games: save browser history and show per-game progress #69.

Starting files

shared/practice/questions.json, src/features/progress/, src/features/practice/domain/session.ts, backend/src/ProCoderJi.Api/Features/Practice/PracticeQuestionBank.cs, and backend/src/ProCoderJi.Api/Features/Progress/.

Done when

Shared fixtures and both runtimes demonstrate that correcting or retiring a question preserves the original attempt's review and result. Tests cover existing unversioned records, revision mismatches, retirement, reload/export/import, cloud synchronization and reset compatibility across supported storage providers.