feat(contests): auto-join user to previously participated contest on …

[ZephyrNova47]

Description

This PR implements an auto-join mechanism for contests
Add new URL route /contest/{key}/problem/{order} for contest problem access
Type of change: feat

What

• Implemented auto-join behavior when a user clicks on a contest link.
• Automatically switches users to an existing participation if they have joined the contest before.
• Add /contest/{key}/problem/{order} route with ContestProblemSubmit view
• Fix: users can now access contest problems without visiting contest home
• Fix: ContestLeave redirects to contest list to prevent auto-rejoin

Why

Fixes # (issue)

How Has This Been Tested?

• Manual UI test
  Scenario 1: User A has already joined the contest and can access the problem directly without needing to visit the contest page first.

Scenario 2: User B has not joined the contest, so accessing the contest URL directly is not allowed.

Checklist

• I have explained the purpose of this PR.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the README/documentation.
• Any dependent changes have been merged and published in downstream modules.
• Informed of breaking changes, testing and migrations (if applicable).
• Attached screenshots (if applicable).

By submitting this pull request, I confirm that my contribution is made under the terms of the AGPL-3.0 License.