refactor: no edits to existing fields

[tlambert03]

this prevents already added fields from being edited in the public forms. at this point, we'd rather not have to re-curate existing data, just receive new data

[codecov]

Codecov Report

Attention: Patch coverage is 43.75000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 48.63%. Comparing base (4af0e95) to head (6bce32a).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
backend/proteins/forms/forms.py	43.75%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #240      +/-   ##
==========================================
+ Coverage   48.56%   48.63%   +0.06%     
==========================================
  Files         113      113              
  Lines        9533     9544      +11     
==========================================
+ Hits         4630     4642      +12     
+ Misses       4903     4902       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.