feat: auto LLM feedback runner + problem link + 5xx retry

- Add SubmissionFeedbackRunner: async background queue for auto LLM feedback
- Enqueue feedback generation after each submission in submitProblem()
- Register runner in main.cc with CSP_FEEDBACK_AUTO_RUN env var
- Add problem_title to GET /api/v1/submissions/{id} response
- Frontend: clickable problem link on submission detail page
- Enhance LLM prompt with richer analysis dimensions
- Add 5xx/connection error retry (max 5 attempts) in Python LLM script

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/src/app/submissions/[id]/page.tsx

@@ -1,5 +1,6 @@
 "use client";
 
+import Link from "next/link";
 import { useParams } from "next/navigation";
 import { useEffect, useMemo, useState } from "react";
 
@@ -21,6 +22,7 @@ type Submission = {
   id: number;
   user_id: number;
   problem_id: number;
+  problem_title?: string;
   contest_id: number | null;
   language: string;
   code: string;
@@ -117,7 +119,17 @@ export default function SubmissionDetailPage() {
           <section className="rounded-xl border bg-white p-4 text-sm">
             <div className="grid gap-1 sm:grid-cols-2">
               <p>{tx("用户", "User")}: {data.user_id}</p>
-              <p>{tx("题目", "Problem")}: {data.problem_id}</p>
+              <p>
+                {tx("题目", "Problem")}:{" "}
+                <Link
+                  href={`/problems/${data.problem_id}`}
+                  className="text-blue-400 hover:text-blue-300 underline"
+                >
+                  {data.problem_title
+                    ? `P${data.problem_id} - ${data.problem_title}`
+                    : `P${data.problem_id}`}
+                </Link>
+              </p>
               <p>{tx("比赛", "Contest")}: {data.contest_id ?? "-"}</p>
               <p>{tx("语言", "Language")}: {data.language}</p>
               <p>{tx("状态", "Status")}: {data.status}</p>