Protect chat mutations with revision compare-and-swap

[dovvnloading]

Summary

• add revision compare-and-swap support to both chat repository implementations
• make in-memory chat mutations lock-protected and SQLite mutations transactional with BEGIN IMMEDIATE
• reject stale synchronous message writes with HTTP 409 and guard generation persistence before both user and assistant writes
• regenerate OpenAPI/TypeScript contracts and add concurrency/race regression coverage

Validation

• python -m pytest -q (372 passed, 1 skipped)
• python -m ruff check ... (all checks passed)
• python tools/generate_contracts.py