feat(api): add endpoint /api/problems

  ## what
  - add endpoint `/api/problems`

  ## how

  ## why
  - this will fetch all problems from upstream

  ## where
  - ./src/app/api/problems/route.ts

  ## usage

src/app/api/problems/route.ts

@@ -0,0 +1,73 @@
+import { uhuntAllProblemsUrl } from "@/utils/constants";
+
+/**
+ * an array of keys that will be used to convert an array of arrays into array of objects
+ *
+ * check `https://uhunt.onlinejudge.org/api/p` to view raw data. (each problem stats are presented as an array element)
+ */
+const arrKey = [
+  "pid",
+  "num",
+  "title",
+  "dacu",
+  "mrun",
+  "mmem",
+  "nover",
+  "sube",
+  "noj",
+  "inq",
+  "ce",
+  "rf",
+  "re",
+  "ole",
+  "tle",
+  "mle",
+  "wa",
+  "pe",
+  "ac",
+  "rtl",
+  "status",
+  "rej",
+];
+
+export const GET = async (_request: Request) => {
+  const url = uhuntAllProblemsUrl();
+
+  const response = await fetch(url);
+  // data returned is an array of array
+  // ex:
+  // [
+  //   [
+  //     36,
+  //     100,
+  //     "The 3n + 1 problem",
+  //     ...
+  //   ],
+  // ]
+  const data = await response.json();
+
+  // convert an array of array into array of problems
+  // ex: 
+  // convert
+  // [
+  //   [1,2,3,...],
+  //   [1,2,3,...],
+  // ]
+  //
+  // into
+  // [
+  //   {pid: 1, num: 2, title: 3, ...},
+  //   {pid: 1, num: 2, title: 3, ...},
+  // ]
+  const converted = data.map((problem: number[]) => {
+    const initialValue = {};
+    return problem.reduce((obj, item, index: number) => {
+      return {
+        ...obj,
+        [arrKey[index]]: item,
+      };
+    }, initialValue);
+  });
+
+  return Response.json(converted);
+};