- Backend emits session id in SSE; frontend sets activeChat so follow-ups
reuse the same session (fixes fragmented history).
- sessions.summary column; messages past RECENT_LIMIT (10) are folded into
a persisted summary via a non-streaming model call.
- buildMessages sends system(+summary) + last 10 messages, bounding context
while preserving long-range memory.
- Tests: aiMessages (capping + summary injection).
