refactor(backend): flatten module structure, move routes to .routes.ts files

- Remove empty controllers/, services/, repositories/ subdirectories
- Move routes/index.ts into module.routes.ts (flat naming convention)
- Update imports in http/app.ts

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

services/backend/src/modules/messages/messages.routes.ts

@@ -0,0 +1,26 @@
+import type { Router } from "express";
+import express from "express";
+import {
+  handleGetAttachmentsByChannel,
+  handleGetMessageById,
+  handleGetMessagesByChannel,
+  handleListMessages,
+} from "./messages.controller.js";
+
+export function createMessagesRouter(): Router {
+  const router = express.Router();
+
+  // GET /api/messages - List messages
+  router.get("/messages", handleListMessages);
+
+  // GET /api/messages/:channelId - Get messages by channel
+  router.get("/messages/:channelId", handleGetMessagesByChannel);
+
+  // GET /api/messages/:channelId/attachments - Get attachments by channel
+  router.get("/messages/:channelId/attachments", handleGetAttachmentsByChannel);
+
+  // GET /api/messages/:id - Get single message by ID
+  router.get("/messages/:id", handleGetMessageById);
+
+  return router;
+}