Fix host upload: handle multer errors as JSON, pre-create upload-tmp dir; v2.0.41

If multer failed to write the temp file (e.g. data-volume permission issue),
the error was propagating unhandled through Express, returning a non-JSON 500
that the client couldn't parse — showing only "Upload failed." with no details.

Changes:
- Import mkdir and pre-create UPLOAD_TEMP_DIR on startup (non-blocking)
- Wrap upload.single('file') in a callback so multer errors return JSON
  responses with the actual message instead of a bare HTML 500

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

server/routes/nuggets-host-upload.js

@@ -1,5 +1,5 @@
 import { randomUUID } from 'node:crypto'
-import { readFile, writeFile, unlink } from 'node:fs/promises'
+import { readFile, writeFile, unlink, mkdir } from 'node:fs/promises'
 import path from 'node:path'
 import multer from 'multer'
 import rateLimit from 'express-rate-limit'
@@ -231,6 +231,10 @@ const otpRateLimiter = rateLimit({
 
 // ── Route registration ────────────────────────────────────────────────────
 export function register(app) {
+  // Ensure temp upload dir exists (non-blocking; created well before any upload)
+  mkdir(UPLOAD_TEMP_DIR, { recursive: true }).catch(err => {
+    console.error('[nuggets-upload] could not create upload-tmp dir:', err.message)
+  })
   // ── Host auth ─────────────────────────────────────────────────────────
 
   app.get('/api/nuggets-upload/me', async (req, res) => {
@@ -389,7 +393,16 @@ export function register(app) {
 
   // ── Episode upload (audio file + metadata) ───────────────────────────
 
-  app.post('/api/nuggets-upload/upload', requireHostAuth, upload.single('file'), async (req, res) => {
+  app.post('/api/nuggets-upload/upload', requireHostAuth, (req, res, next) => {
+    upload.single('file')(req, res, err => {
+      if (!err) return next()
+      if (err.code === 'LIMIT_FILE_SIZE') {
+        return res.status(400).json({ message: 'File exceeds the 500 MB limit.' })
+      }
+      console.error('[nuggets-upload] multer error:', err.message)
+      return res.status(500).json({ message: `File upload error: ${err.message}` })
+    })
+  }, async (req, res) => {
     if (!req.file) {
       res.status(400).json({ message: 'No file received.' })
       return