feat(problem): add IP-auth submission API endpoint

[whlongg]

Description

Type of change: new feature

What

• Add a CSRF-exempt /problem/<code>/api_submit view for trusted IP-authenticated clients, reusing the existing submission pipeline while returning JSON.
• Introduce language-extension preference mapping and a helper to pick the best match when inferring the language from uploaded files.
• Inline non–file-only submissions as source text, leaving file uploads only for file-only languages, so API judging mirrors the UI flow.
• Register the new endpoint in site/dmoj/urls.py without affecting the classic /submit.

Why

To provide an official form-data submission API for IP-based authentication users while keeping the current UI behaviour untouched.

Fixes #

How Has This Been Tested?

• Test A – Manual UI submission (browser) succeeds and still requires CSRF.
• Test B – curl -F submission_file=@aplusb.cpp http://0.0.0.0:8000/problem/aplusb/api_submit from an IP-auth session returns {"id": ...} and judges correctly; the same request from a non IP-auth session returns the expected JSON 403.

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.

[whlongg]

@leduythuccs The branch has been updated.