fix(pdf-compress): fix broken multipart parsing + add body size limit

Extreme mode: replace fragile manual multipart boundary parsing (which
extracted only a fraction of large files, producing empty PDFs) with
standard req.formData(). Add GS output validation + stderr capture.

Stirling mode: parse formData first then build fresh FormData for
Stirling instead of raw body passthrough (which lost data on large
files). Add 5min timeout + original/compressed size headers.

next.config: add 250MB body size limit for server actions.

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

next.config.ts

@@ -2,6 +2,11 @@ import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
   output: 'standalone',
+  experimental: {
+    serverActions: {
+      bodySizeLimit: '250mb',
+    },
+  },
 };
 
 export default nextConfig;