fix: Improve perf random logo search

[raphael0202]

Fixes #1386.

We use the newly indexed column logo_annotation.server_type to filter logos of a specific server_type.
Note that the sampling fraction we've chosen (20%) may not be adapted depending on how infrequent a certain server_type is. Currently, we only have logos with server_type=off, so we can keep it this way.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 56.06%. Comparing base (a318737) to head (7f65d10).

Files	Patch %	Lines
robotoff/app/api.py	0.00%	4 Missing ⚠️
robotoff/cli/main.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1397      +/-   ##
==========================================
- Coverage   56.06%   56.06%   -0.01%     
==========================================
  Files          80       80              
  Lines        8010     8011       +1     
==========================================
  Hits         4491     4491              
- Misses       3519     3520       +1     

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