refactor: rename middleware to proxy for Next 16

Next 16 deprecates the middleware file convention; a leftover
middleware.js is silently ignored, which would break the x-pathname
header that canonical URL generation depends on.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

proxy.js

@@ -1,6 +1,6 @@
 import { NextResponse } from 'next/server';
 
-export function middleware(request) {
+export function proxy(request) {
     const response = NextResponse.next();
 
     // Add pathname to headers so we can access it in getCanonicalUrl