fix(github-oauth): use NEXTAUTH_URL for redirect_uri instead of req.url.origin

Behind Coolify's proxy, req.url resolves to 0.0.0.0:3000 which GitHub
rejects as an unregistered redirect URI. Prefer NEXTAUTH_URL env var.

Made-with: Cursor

app/api/integrations/github/callback/route.ts

@@ -32,7 +32,7 @@ function bounce(origin: string, returnTo: string, params: Record<string, string>
 
 export async function GET(req: Request) {
   const url = new URL(req.url);
-  const origin = url.origin;
+  const origin = (process.env.NEXTAUTH_URL ?? url.origin).replace(/\/$/, "");
 
   // Recover the original target from the state cookie *before* any error path.
   const cookieState = req.headers.get("cookie")