feat: rolling conversation summary + session continuity

- 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).

supabase/migrations/20260819150000_add_sessions_summary.sql

@@ -0,0 +1,6 @@
+-- Add a rolling conversation summary column to sessions.
+-- Older messages get folded into this summary so the AI context stays bounded
+-- (model receives summary + last N messages) while long-range context persists.
+
+ALTER TABLE public.sessions
+  ADD COLUMN IF NOT EXISTS summary text;