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).

frontend/src/App.jsx

@@ -347,7 +347,11 @@ function App() {
 
               try {
                 const data = JSON.parse(jsonStr);
-                if (data.type === "chunk") {
+                if (data.type === "session") {
+                  // Remember which session this conversation belongs to so
+                  // follow-up messages reuse it (history stays together).
+                  setActiveChat(data.id);
+                } else if (data.type === "chunk") {
                   assistantText += data.content;
                   setMessages((prev) =>
                     prev.map((m) =>