feat(22-01): add SSE streaming endpoint and edit/truncate service methods

- Add editMessage, truncateMessagesAfter, streamEcho methods to chatService
- Add POST /conversations/:id/stream SSE endpoint with flushHeaders before loop (PERF-02)
- Add PATCH /conversations/:id/messages/:msgId for message editing
- Add DELETE /conversations/:id/messages/after/:msgId for message truncation
- Import gt from drizzle-orm for createdAt comparison in truncateMessagesAfter
- Guard all res.write() calls with res.writable check (prevents write-after-end)

server/src/routes/chat.ts

@@ -77,5 +77,72 @@ export function chatRoutes(db: Db): Router {
     res.status(201).json(message);
   });
 
+  // POST /api/conversations/:id/stream -- SSE streaming endpoint (CHAT-01, PERF-02)
+  router.post("/conversations/:id/stream", async (req, res) => {
+    assertBoard(req);
+    const { content, agentId } = req.body;
+    if (!content || typeof content !== "string") {
+      res.status(400).json({ error: "content is required" });
+      return;
+    }
+
+    // Set SSE headers and flush BEFORE any generation (PERF-02)
+    res.setHeader("Content-Type", "text/event-stream");
+    res.setHeader("Cache-Control", "no-cache");
+    res.setHeader("Connection", "keep-alive");
+    res.setHeader("X-Accel-Buffering", "no");
+    res.flushHeaders();
+    res.write(":ok\n\n");
+
+    const abort = new AbortController();
+    req.on("close", () => abort.abort());
+
+    try {
+      let fullContent = "";
+      for await (const token of svc.streamEcho(content, abort.signal)) {
+        if (!res.writable) break;
+        fullContent += token;
+        res.write(`data: ${JSON.stringify({ token })}\n\n`);
+      }
+      if (res.writable && !abort.signal.aborted) {
+        const message = await svc.addMessage(req.params.id!, {
+          role: "assistant",
+          content: fullContent.trim(),
+          agentId: agentId || undefined,
+        });
+        res.write(`data: ${JSON.stringify({ done: true, messageId: message.id, content: fullContent.trim() })}\n\n`);
+      }
+    } catch (err) {
+      if (res.writable && !abort.signal.aborted) {
+        res.write(`data: ${JSON.stringify({ error: "Stream error" })}\n\n`);
+      }
+    } finally {
+      res.end();
+    }
+  });
+
+  // PATCH /api/conversations/:id/messages/:msgId -- Edit message content
+  router.patch("/conversations/:id/messages/:msgId", async (req, res) => {
+    assertBoard(req);
+    const { content } = req.body;
+    if (!content || typeof content !== "string") {
+      res.status(400).json({ error: "content is required" });
+      return;
+    }
+    const message = await svc.editMessage(req.params.msgId!, content);
+    if (!message) {
+      res.status(404).json({ error: "Message not found" });
+      return;
+    }
+    res.json(message);
+  });
+
+  // DELETE /api/conversations/:id/messages/after/:msgId -- Truncate messages after a given message
+  router.delete("/conversations/:id/messages/after/:msgId", async (req, res) => {
+    assertBoard(req);
+    await svc.truncateMessagesAfter(req.params.id!, req.params.msgId!);
+    res.status(204).end();
+  });
+
   return router;
 }

server/src/services/chat.ts

@@ -1,4 +1,4 @@
-import { and, desc, eq, ilike, isNull, lt } from "drizzle-orm";
+import { and, desc, eq, gt, ilike, isNull, lt } from "drizzle-orm";
 import type { Db } from "@paperclipai/db";
 import { chatConversations, chatMessages } from "@paperclipai/db";
 import { notFound } from "../errors.js";
@@ -165,5 +165,40 @@ export function chatService(db: Db) {
 
       return message!;
     },
+
+    async editMessage(messageId: string, content: string) {
+      const [row] = await db
+        .update(chatMessages)
+        .set({ content })
+        .where(eq(chatMessages.id, messageId))
+        .returning();
+      return row;
+    },
+
+    async truncateMessagesAfter(conversationId: string, messageId: string) {
+      // Get the target message's createdAt
+      const [target] = await db
+        .select({ createdAt: chatMessages.createdAt })
+        .from(chatMessages)
+        .where(eq(chatMessages.id, messageId));
+      if (!target) return;
+      await db
+        .delete(chatMessages)
+        .where(
+          and(
+            eq(chatMessages.conversationId, conversationId),
+            gt(chatMessages.createdAt, target.createdAt),
+          ),
+        );
+    },
+
+    async *streamEcho(content: string, signal: AbortSignal) {
+      const words = content.split(/\s+/);
+      for (const word of words) {
+        if (signal.aborted) break;
+        await new Promise((r) => setTimeout(r, 50));
+        yield word + " ";
+      }
+    },
   };
 }