fix(auth): resolve cookie authentication failure over HTTP

Cookies were set with secure flag based solely on NODE_ENV, causing
401 errors when accessing over HTTP with NODE_ENV=production.

- Add COOKIE_SECURE env var for explicit control
- Auto-detect HTTPS via X-Forwarded-Proto header in production
- Extract isSecureCookie() utility to lib/auth/utils.ts
- Document COOKIE_SECURE in README and .env.example

Fixes #39

app/api/auth/login/route.ts

@@ -1,5 +1,5 @@
 import { NextRequest, NextResponse } from 'next/server';
-import { generateAccessToken, generateRefreshToken, validateAdminCredentials } from '@/lib/auth/utils';
+import { generateAccessToken, generateRefreshToken, validateAdminCredentials, isSecureCookie } from '@/lib/auth/utils';
 
 export async function POST(request: NextRequest) {
   try {
@@ -36,7 +36,7 @@ export async function POST(request: NextRequest) {
     // Set cookies
     const cookieOptions = {
       httpOnly: true,
-      secure: process.env.NODE_ENV === 'production',
+      secure: isSecureCookie(request),
       sameSite: 'lax' as const,
       path: '/',
     };