When a user switches models mid-session via /model, the gateway updates
the in-memory agent and session overrides, but the database was never
updated. The COALESCE(model, ?) in update_token_counts() only fills NULL
values, so the dashboard always showed the original model.
Fix: Add SessionDB.update_session_model() that unconditionally sets the
model column, and call it from both the interactive picker and direct
/model command paths in the gateway.
Fixes#34850
