Run migrations automatically on app startup

[dmjb]

Also support two separate DB users: A migration user, and a regular user.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 23.25581% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.56%. Comparing base (4ffe3f6) to head (fc4400f).

Files with missing lines	Patch %	Lines
internal/config/config.go	0.00%	33 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #145      +/-   ##
==========================================
- Coverage   67.19%   66.56%   -0.63%     
==========================================
  Files          60       60              
  Lines        3707     3742      +35     
==========================================
  Hits         2491     2491              
- Misses       1062     1096      +34     
- Partials      154      155       +1     

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[blkt]

I did the following to ensure that everything was working fine

• added a test running the migration process as serve subcommand does concurrently a bunch of times (default is 3 for the test, I ran 100 parallel instances of it and none failed)
• manually tested by running 10 instances of the process at the same time providing the same configuration (except for the address)
• changed the migration so that tests failed to ensure that they were successfully catching weird scenarios (they do)
• document the sole assumptions we make about what the DB/Infra admin needs to provide in README.md