feat: add backend Express server with API and static serving

backend/src/index.ts

@@ -0,0 +1,26 @@
+import express from "express";
+import cors from "cors";
+import path from "path";
+import imageRoutes from "./routes/image";
+
+const app = express();
+const port = process.env.PORT || 3000;
+
+app.use(cors());
+app.use(express.json());
+
+// API routes
+app.use("/api", imageRoutes);
+
+// Serve frontend static build
+const publicDir = path.join(__dirname, "public");
+app.use(express.static(publicDir));
+
+// Fallback to index.html for SPA routing
+app.get("*", (_req, res) => {
+  res.sendFile(path.join(publicDir, "index.html"));
+});
+
+app.listen(port, () => {
+  console.log(`Server listening on port ${port}`);
+});