add first photo in observation API lists, change thumbnails to JPEG/70

[submarcos]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.

Project coverage is 83.33%. Comparing base (39c1b64) to head (b0587ee).

Files	Patch %	Lines
backend/project/api/views.py	50.00%	1 Missing ⚠️
backend/project/observations/models.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             next      #12      +/-   ##
==========================================
+ Coverage   83.28%   83.33%   +0.04%     
==========================================
  Files          16       16              
  Lines         353      360       +7     
  Branches       20       21       +1     
==========================================
+ Hits          294      300       +6     
- Misses         57       58       +1     
  Partials        2        2              

Flag	Coverage Δ
python-unittests	83.33% <85.71%> (+0.04%)	⬆️

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