fix(backend): CORS and API URL fixes

frontend/src/lib/api.ts

@@ -9,7 +9,11 @@ import type {
   SubmissionResponse,
 } from "@/types";
 
-const API_BASE_URL = process.env.NEXT_PUBLIC_API_URL || "http://localhost:8000";
+// Server-side uses internal Docker network, client-side uses public URL
+const API_BASE_URL =
+  typeof window === "undefined"
+    ? process.env.API_URL || "http://localhost:8000"
+    : process.env.NEXT_PUBLIC_API_URL || "http://localhost:8000";
 
 export class ApiError extends Error {
   constructor(