feat: implement Story 3.1 — chat panel UI with streaming AI responses

Add AI copilot chat panel to the diagram editor with streaming responses,
chat history persistence, and markdown rendering. Includes copilot API route,
diagram-aware system prompt, and schema with 15 passing tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/api/src/modules/ai/copilot/router.ts

@@ -0,0 +1,47 @@
+import { Hono } from "hono";
+import * as z from "zod";
+
+import { getCopilotHistory, streamCopilot } from "@turbostarter/ai/copilot/api";
+import { copilotMessageSchema } from "@turbostarter/ai/copilot/schema";
+
+import { enforceAuth, deductCredits, rateLimiter, validate } from "../../../middleware";
+
+import type { User } from "@turbostarter/auth";
+
+const chatIdQuerySchema = z.object({
+  chatId: z.string(),
+});
+
+export const copilotRouter = new Hono<{
+  Variables: {
+    user: User;
+  };
+}>()
+  .get(
+    "/messages",
+    enforceAuth,
+    validate("query", chatIdQuerySchema),
+    async (c) => {
+      const { chatId } = c.req.valid("query");
+      const messages = await getCopilotHistory(chatId);
+      return c.json(messages);
+    },
+  )
+  .post(
+    "/",
+    enforceAuth,
+    rateLimiter,
+    validate("json", copilotMessageSchema),
+    async (c) => {
+      const input = c.req.valid("json");
+
+      // Deduct 1 credit per copilot message
+      await deductCredits(1, "copilot")(c, async () => { /* noop */ });
+
+      return streamCopilot({
+        ...input,
+        signal: c.req.raw.signal,
+        userId: c.var.user.id,
+      });
+    },
+  );

packages/api/src/modules/ai/router.ts

@@ -5,6 +5,7 @@ import { getUserCredits } from "@turbostarter/ai/credits/server";
 import { enforceAuth } from "../../middleware";
 
 import { chatRouter } from "./chat";
+import { copilotRouter } from "./copilot/router";
 import { imageRouter } from "./image";
 import { pdfRouter } from "./pdf";
 import { sttRouter } from "./stt";
@@ -13,6 +14,7 @@ import { ttsRouter } from "./tts";
 export const aiRouter = new Hono()
   .use(enforceAuth)
   .route("/chat", chatRouter)
+  .route("/copilot", copilotRouter)
   .route("/pdf", pdfRouter)
   .route("/image", imageRouter)
   .route("/tts", ttsRouter)