feat: add "db merge" subcommand

[tedil]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 22.22222% with 63 lines in your changes missing coverage. Please review.

Project coverage is 73.86%. Comparing base (d1f8f8d) to head (29da1b0).
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/db/merge.rs	21.51%	62 Missing ⚠️
src/main.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #517      +/-   ##
==========================================
- Coverage   74.01%   73.86%   -0.16%     
==========================================
  Files          25       26       +1     
  Lines        9840     9860      +20     
==========================================
  Hits         7283     7283              
- Misses       2557     2577      +20     

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