fix(persistence): connection pool leak due to schema migrations

[michaeladler]

Description

Previously, a separate connection pool was created to execute schema migrations. This pool was not properly shut down though, leading to open connections and unused goroutines. This commit reuses the connection pool created for entgo, thereby preventing any resource leaks.

Issues Addressed

List and link all the issues addressed by this PR.

Change Type

Please select the relevant options:

• Bug fix (non-breaking change that resolves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• I have read the CONTRIBUTING document.
• My changes adhere to the established code style, patterns, and best practices.
• I have added tests that demonstrate the effectiveness of my changes.
• I have updated the documentation accordingly (if applicable).
• I have added an entry in the CHANGELOG to document my changes (if applicable).

[codecov]

Codecov Report

Merging #29 (2b0631a) into main (2e54f45) will increase coverage by 0.26%.
The diff coverage is 54.54%.

@@            Coverage Diff             @@
##             main      #29      +/-   ##
==========================================
+ Coverage   77.70%   77.97%   +0.26%     
==========================================
  Files          80       80              
  Lines        2983     2996      +13     
==========================================
+ Hits         2318     2336      +18     
+ Misses        487      481       -6     
- Partials      178      179       +1     

Files Changed	Coverage Δ
internal/persistence/entgo/sqlite.go	54.28% <53.57%> (-0.89%)	⬇️
internal/persistence/entgo/mysql.go	64.00% <56.25%> (+8.18%)	⬆️

... and 2 files with indirect coverage changes