Use type-safe queryset methods instead of custom ORM queries

[jodal]

This improves code readability and makes the code more type-safe at the same time.

[codecov]

Codecov Report

❌ Patch coverage is 56.80000% with 54 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.85%. Comparing base (def70ce) to head (bd109c9).
⚠️ Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
src/comics/browser/views.py	9.09%	30 Missing ⚠️
src/comics/core/querysets.py	76.47%	12 Missing ⚠️
src/comics/accounts/views.py	16.66%	5 Missing ⚠️
src/comics/aggregator/downloader.py	0.00%	4 Missing ⚠️
src/comics/accounts/querysets.py	80.00%	1 Missing ⚠️
src/comics/api/views.py	94.73%	1 Missing ⚠️
src/comics/status/views.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #674      +/-   ##
==========================================
+ Coverage   58.91%   59.85%   +0.93%     
==========================================
  Files          41       41              
  Lines        2013     2035      +22     
==========================================
+ Hits         1186     1218      +32     
+ Misses        827      817      -10     

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