Replace full DELETE/INSERT with efficient delta writes:
- Track db_message_count on Session (how many rows already persisted).
- On save(): UPDATE mutable flags for existing rows, DELETE only extras
  (race guard), INSERT new messages via executemany.
- Reduces DB write amplification from O(N) to O(delta) per turn.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>