Add schema tests, change migration in tests to always migrate to latest

[rcmonsayac]

Changes

• Add schema tests for post, author, tag, and publication
• Make migration a single file and depend on @current_version in postgres.ex to always migrate to the latest version similar to oban

[codecov]

Codecov Report

Attention: Patch coverage is 16.66667% with 5 lines in your changes missing coverage. Please review.

Project coverage is 74.27%. Comparing base (d2e943d) to head (538e7b2).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/literature/migrations/postgres/v02.ex	0.00%	4 Missing ⚠️
lib/literature/migrations/postgres/v06.ex	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #378      +/-   ##
==========================================
+ Coverage   74.25%   74.27%   +0.01%     
==========================================
  Files          74       75       +1     
  Lines        1954     1955       +1     
==========================================
+ Hits         1451     1452       +1     
  Misses        503      503              

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