Add 2d curvedata model

[tlambert03]

This would capture full curve data for PKA, Bleaching , and maturation curves

[codecov]

Codecov Report

❌ Patch coverage is 70.22901% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.83%. Comparing base (1979c1d) to head (c4d85fd).

Files with missing lines	Patch %	Lines
backend/proteins/models/curve.py	61.38%	39 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #405      +/-   ##
==========================================
+ Coverage   62.70%   62.83%   +0.12%     
==========================================
  Files         115      116       +1     
  Lines        9202     9333     +131     
==========================================
+ Hits         5770     5864      +94     
- Misses       3432     3469      +37     

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