Add Fastify backend — static serving and save API

server/index.js

@@ -0,0 +1,71 @@
+import Fastify from 'fastify'
+import fastifyStatic from '@fastify/static'
+import { readFile, writeFile, mkdir } from 'fs/promises'
+import { join, dirname } from 'path'
+import { fileURLToPath } from 'url'
+import { existsSync } from 'fs'
+
+const __dirname = dirname(fileURLToPath(import.meta.url))
+const ROOT = join(__dirname, '..')
+const DIST = join(ROOT, 'dist')
+const SAVES_DIR = join(ROOT, 'data', 'saves')
+
+// Ensure saves directory exists
+await mkdir(SAVES_DIR, { recursive: true })
+
+const fastify = Fastify({ logger: false })
+
+// Serve built frontend
+await fastify.register(fastifyStatic, {
+  root: DIST,
+  prefix: '/',
+})
+
+// Load save state
+fastify.get('/api/save/:token', async (req, reply) => {
+  const { token } = req.params
+  if (!/^[0-9a-f]{8}$/.test(token)) {
+    return reply.code(400).send({ error: 'Invalid token' })
+  }
+  const path = join(SAVES_DIR, `${token}.json`)
+  if (!existsSync(path)) {
+    return reply.code(404).send({ error: 'Save not found' })
+  }
+  try {
+    const raw = await readFile(path, 'utf8')
+    return reply.send(JSON.parse(raw))
+  } catch {
+    return reply.code(500).send({ error: 'Failed to read save' })
+  }
+})
+
+// Write save state
+fastify.post('/api/save/:token', async (req, reply) => {
+  const { token } = req.params
+  if (!/^[0-9a-f]{8}$/.test(token)) {
+    return reply.code(400).send({ error: 'Invalid token' })
+  }
+  if (!req.body || typeof req.body !== 'object') {
+    return reply.code(400).send({ error: 'Invalid body' })
+  }
+  const path = join(SAVES_DIR, `${token}.json`)
+  try {
+    await writeFile(path, JSON.stringify(req.body, null, 2), 'utf8')
+    return reply.send({ ok: true })
+  } catch {
+    return reply.code(500).send({ error: 'Failed to write save' })
+  }
+})
+
+// Fallback — serve index.html for client-side routing
+fastify.setNotFoundHandler((req, reply) => {
+  reply.sendFile('index.html')
+})
+
+try {
+  await fastify.listen({ port: 3000, host: '0.0.0.0' })
+  console.log('OTIVM server running on port 3000')
+} catch (err) {
+  console.error(err)
+  process.exit(1)
+}