refactor: migrate deprecated middleware convention to proxy (Next 16)

Middlewate file convention is deprecated in Next 16.3.5 (renamed to proxy).
git mv middleware.ts -> proxy.ts, rename exported fn middleware -> proxy.
Build now emits `ƒ Proxy (Middleware)` with no deprecation warning.

.github/dependabot.yml

@@ -48,4 +48,4 @@ updates:
         patterns: ["*"]
         update-types:
           - "minor"
-          - "patch"
+          - "patch"

proxy.ts

@@ -3,7 +3,7 @@ import { getSessionCookie } from "better-auth/cookies";
 
 const PROTECTED_PATHS = ["/protected"];
 
-export function middleware(request: NextRequest) {
+export function proxy(request: NextRequest) {
   const { pathname } = request.nextUrl;
 
   if (PROTECTED_PATHS.some((p) => pathname.startsWith(p))) {