fix: 405 instead of 501 on POST to v1 detail API

[jennifer-richards]

This will prevent 501 errors on POST to API v1 routes like /api/v1/submit/submission/auth/. Note that there's nothing special about that app/model or the /auth/ there - we get the same response for /api/v1/doc/document/frobozz/zorkmid/.

This is a narrow fix, specifically handling the post_detail view. We could consider wholesale blocking of POST/PATCH/PUT/DELETE to the v1 API, but that will require touching all of our resource classes to limit the allowed methods (at least for the straightforward mechanism, there may be cleverer approaches).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.58%. Comparing base (8dbc1bc) to head (32c0dfc).
⚠️ Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11162      +/-   ##
==========================================
+ Coverage   88.52%   88.58%   +0.06%     
==========================================
  Files         334      333       -1     
  Lines       45102    44665     -437     
==========================================
- Hits        39927    39568     -359     
+ Misses       5175     5097      -78     

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