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).

backend/src/routes/chat.js

@@ -1,6 +1,6 @@
 import express from "express";
-import { streamChatResponse } from "../services/ai.js";
-import { createSession, addMessage, getMessages, getSession, listSessions } from "../services/db.js";
+import { RECENT_LIMIT, streamChatResponse, summarizeConversation } from "../services/ai.js";
+import { createSession, addMessage, getMessages, getSession, listSessions, setSummary } from "../services/db.js";
 import { isOwnedSession, validateChatInput } from "./chatValidation.js";
 
 const router = express.Router();
@@ -33,6 +33,23 @@ router.post("/", async (req, res) => {
     res.setHeader("Connection", "keep-alive");
     res.setHeader("X-Accel-Buffering", "no");
 
+    // Tell the client which session this conversation belongs to, so follow-ups
+    // reuse the same session and history accumulates.
+    res.write(`data: ${JSON.stringify({ type: "session", id: currentChatId })}\n\n`);
+
+    // Bounded context: fold older messages into a persistent summary, keep the
+    // last RECENT_LIMIT raw.
+    let summary = session.summary || "";
+    if (chatMessages.length > RECENT_LIMIT) {
+      const overflow = chatMessages.slice(0, chatMessages.length - RECENT_LIMIT);
+      try {
+        summary = await summarizeConversation(session.summary, overflow);
+        await setSummary(currentChatId, summary);
+      } catch (err) {
+        console.error("Summarize error:", err.message);
+      }
+    }
+
     let assistantText = "";
     await streamChatResponse(
       chatMessages,
@@ -50,7 +67,8 @@ router.post("/", async (req, res) => {
       () => {
         res.write(`data: ${JSON.stringify({ type: "error", message: "AI service error" })}\n\n`);
         res.end();
-      }
+      },
+      summary
     );
 
     res.write(`data: ${JSON.stringify({ type: "done" })}\n\n`);