fix: populate topic author in message list (#6)

## Summary

<!-- What does this PR change and why? -->

## OpenSpec

- [ ] This PR implements an OpenSpec change: `<!-- change name -->`
- [ ] No OpenSpec change is needed (small doc/fix, no shipped behavior
change).

## Verification

- [ ] `pnpm verify` passed.
- [ ] A subset was run instead: `<!-- commands -->` because `<!-- reason
-->`.

## Impact Checklist

- [ ] App code (`apps/`, `packages/`)
- [ ] API behavior / OpenAPI contract (`apps/api/src/routes/*.ts`
zod-openapi declarations, `api/openapi.json` via `pnpm gen:openapi`)
- [ ] Database schema or migration (`packages/db`)
- [ ] Deployment (`deployment/`)
- [ ] Documentation (`README.md`, `docs/`, `wiki/`)
- [ ] CI / scripts (`.github/workflows/`, `scripts/`, `package.json`)
- [ ] Secret handling: no real `.env`, tokens, cookies, private keys,
database URLs, or user data committed.

## Migration / Deployment Notes

<!-- If this PR affects schema, data migration, or production
deployment, describe the runbook steps. Otherwise write "None". -->

Author: fengmk2

apps/api/src/lib/message.ts

@@ -100,10 +100,12 @@ export async function getMessageRelations(msg: typeof messages.$inferSelect) {
     msg.topicId ? topicQueries.getById(msg.topicId) : null,
     msg.replyId ? replyQueries.getById(msg.replyId) : null,
   ]);
+  const topicAuthor = topic ? await userQueries.getById(topic.authorId) : null;
+
   return {
     ...msg,
     author,
-    topic,
+    topic: topic ? { ...topic, author: topicAuthor } : null,
     reply,
     isInvalid: !author || !topic,
   };

apps/api/src/routes/message.ts

@@ -75,7 +75,9 @@ message.openapi(listMessagesRoute, async (c) => {
       author: { loginname: author.loginname, avatar_url: author.avatar ?? "" },
       topic: {
         id: String(topic.id),
-        author: { loginname: "", avatar_url: "" },
+        author: topic.author
+          ? { loginname: topic.author.loginname, avatar_url: topic.author.avatar ?? "" }
+          : { loginname: "", avatar_url: "" },
         title: topic.title ?? "",
         last_reply_at: topic.lastReplyAt ? topic.lastReplyAt.toISOString() : null,
       },

apps/api/test/message-relations.test.ts

@@ -0,0 +1,67 @@
+import { beforeEach, describe, expect, it, vi } from "vite-plus/test";
+
+const mocks = vi.hoisted(() => ({
+  userGetById: vi.fn(),
+  topicGetById: vi.fn(),
+  replyGetById: vi.fn(),
+}));
+
+vi.mock("../src/lib/db", () => ({
+  getDb: vi.fn(),
+  userQueries: { getById: mocks.userGetById },
+  topicQueries: { getById: mocks.topicGetById },
+  replyQueries: { getById: mocks.replyGetById },
+}));
+
+vi.mock("../src/lib/mail", () => ({
+  sendAtNotifyMail: vi.fn(),
+  sendReplyNotifyMail: vi.fn(),
+}));
+
+import { getMessageRelations } from "../src/lib/message";
+
+const messageRow = {
+  id: 1,
+  type: "reply",
+  masterId: 10,
+  authorId: 20,
+  topicId: 2,
+  replyId: null,
+  hasRead: false,
+  createAt: new Date("2026-08-05T00:00:00.000Z"),
+};
+
+const actor = { id: 20, loginname: "alice", avatar: "https://example.com/alice.png" };
+const topicAuthor = { id: 30, loginname: "bob", avatar: "https://example.com/bob.png" };
+const topic = { id: 2, title: "话题标题", authorId: 30, lastReplyAt: null };
+
+describe("getMessageRelations", () => {
+  beforeEach(() => {
+    mocks.userGetById.mockReset();
+    mocks.topicGetById.mockReset();
+    mocks.replyGetById.mockReset();
+    mocks.userGetById.mockImplementation(async (id: number) => {
+      if (id === actor.id) return actor;
+      if (id === topicAuthor.id) return topicAuthor;
+      return null;
+    });
+    mocks.topicGetById.mockResolvedValue(topic);
+    mocks.replyGetById.mockResolvedValue(null);
+  });
+
+  it("attaches the topic author so /messages can render it", async () => {
+    const relations = await getMessageRelations(messageRow);
+
+    expect(relations.author).toEqual(actor);
+    expect(relations.topic?.author).toEqual(topicAuthor);
+  });
+
+  it("keeps topic null and flags invalid when the topic is missing", async () => {
+    mocks.topicGetById.mockResolvedValue(null);
+
+    const relations = await getMessageRelations({ ...messageRow, topicId: null });
+
+    expect(relations.topic).toBeNull();
+    expect(relations.isInvalid).toBe(true);
+  });
+});