remove client-side snapgene

[tlambert03]

client side snapgene link fetching is flaky, so this just adds a database model, which will get synched by a job periodically

[codecov]

Codecov Report

❌ Patch coverage is 22.48062% with 100 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.55%. Comparing base (7c27eaf) to head (82d2a4b).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...eins/management/commands/sync_snapgene_plasmids.py	0.00%	95 Missing ⚠️
backend/proteins/admin.py	78.57%	3 Missing ⚠️
backend/proteins/models/snapgene.py	88.88%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #378      +/-   ##
==========================================
- Coverage   61.08%   60.55%   -0.54%     
==========================================
  Files         108      110       +2     
  Lines        8694     8823     +129     
==========================================
+ Hits         5311     5343      +32     
- Misses       3383     3480      +97     

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