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>

docker-compose.yml

@@ -22,6 +22,9 @@ services:
       - CSP_SOLUTION_AUTO_RUN_MISSING=true
       - CSP_SOLUTION_AUTO_LIMIT=50000
       - CSP_SOLUTION_AUTO_MAX_SOLUTIONS=3
+      - CSP_FEEDBACK_AUTO_RUN=true
+      - CSP_FEEDBACK_AUTO_LIMIT=500
+      - CSP_FEEDBACK_SCRIPT_PATH=/app/scripts/analyze_submission_feedback.py
     build:
       context: .
       dockerfile: Dockerfile.backend