Add postinstall build step for Clever Cloud

Clever Cloud only runs install + start, not a separate build phase.
Run next build in postinstall so .next/ exists at start time.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Author: hubyrod

package.json

@@ -3,6 +3,7 @@
   "type": "module",
   "private": true,
   "scripts": {
+    "postinstall": "next build",
     "dev": "next dev -H 0.0.0.0 -p ${PORT:-8080}",
     "build": "next build",
     "start": "next start -H 0.0.0.0 -p ${PORT:-8080}",