session: extend List response with summary, turn_count, token totals, model

Add five aggregate fields to Session struct (Summary, TurnCount,
TokensInTotal, TokensOutTotal, Model) populated via a new
sessionListSelectColumns const that wraps correlated subqueries.
List uses the new const; Get is unchanged.

TestList_Aggregates covers: zero turns, long-content truncation to 200
chars, mixed-role model tracking (newest turn wins), and NULL token sums.

feat(session): list and detail JSON API with filters