Switch season rank API to user achievement endpoint

[ToxicBiohazard]

Types of changes

What types of changes does your code introduce?
Put an x in the boxes that apply.

• Bugfix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality not to work as expected).
• Documentation Update (if none of the other choices applies).

Proposed changes

Update get_season_rank to call v4/user/achievement/season/{userId}/{modelId} instead of the old v4/season/end/{seasonId}/{userId} endpoint.

Per @shad0wn0mad (platform), we were asked to switch to this endpoint; they confirmed the response schema is the same, so existing rank/tier parsing is unchanged.

Also adds unit coverage for the new URL and empty-data handling.

Checklist

Put an x in the boxes that apply.

• I have read and followed the CONTRIBUTING.md
  doc.
• Lint and unit tests pass locally with my changes.
• I have added the necessary documentation (if appropriate).

Additional context

• Old: {API_V4_URL}/season/end/{SEASON_ID}/{htb_uid}
• New: {API_V4_URL}/user/achievement/season/{htb_uid}/{SEASON_ID}

Made with Cursor

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.60%. Comparing base (1abfd4b) to head (7de98db).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #192      +/-   ##
==========================================
+ Coverage   66.54%   66.60%   +0.06%     
==========================================
  Files          54       54              
  Lines        3177     3177              
==========================================
+ Hits         2114     2116       +2     
+ Misses       1063     1061       -2     

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[dimoschi]

LGTM