refactor(odeme): update environment variable usage and documentation

Replaced `NEXT_PUBLIC_APP_URL` with `APP_URL` in the payment processing logic to ensure proper runtime access. Updated related documentation to clarify the distinction between build-time and runtime environment variables, emphasizing the importance of using `APP_URL` for callback and webhook URLs. Adjusted references in the codebase to reflect this change, enhancing clarity and functionality in the payment flow.

src/app/api/odeme/callback/route.ts

@@ -1,5 +1,6 @@
 import { eq } from "drizzle-orm";
 import { NextResponse, type NextRequest } from "next/server";
+import { appUrl as siteKok } from "@/lib/app-url";
 import { appDb, schema } from "@/lib/appdb";
 import { odemeyiSonuclandir } from "@/lib/odeme";
 
@@ -11,7 +12,7 @@ import { odemeyiSonuclandir } from "@/lib/odeme";
 // SameSite=Lax nedeniyle session çerezi GELMEZ — kullanıcı token'dan çözülür,
 // verifySession ÇAĞRILMAZ. Kredi tanımlama odemeyiSonuclandir içinde idempotenttir.
 export async function POST(request: NextRequest) {
-  const appUrl = process.env.NEXT_PUBLIC_APP_URL ?? "http://localhost:3000";
+  const appUrl = siteKok();
 
   let token: string | undefined;
   try {