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>

backend/CMakeLists.txt

@@ -28,6 +28,7 @@ add_library(csp_core
   src/services/kb_import_runner.cc
   src/services/problem_gen_runner.cc
   src/services/submission_feedback_service.cc
+  src/services/submission_feedback_runner.cc
   src/services/import_service.cc
   src/services/import_runner.cc
   src/domain/enum_strings.cc