Replaced `if not session.context:` with a role-based check so the system
message is inserted whenever it is absent — not just for brand-new sessions.

Root cause: backward-compat sessions (context column was empty) had their
context initialised from session.messages, which never contains a system
message. The old check (`if not session.context:`) saw a non-empty list and
skipped the system prompt, so every subsequent request ran without it —
Navi had no persona and no profile instructions.

Also add context_token_count field to Session (follow-up for token counter
fix — persistence wiring comes in next commit).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>