Remove duplicate Cursor implementation, query code clean up

[adzialocha]

Closes #344

📋 Checklist

• Add tests that cover your changes
• Add this PR to the Unreleased section in CHANGELOG.md
• Link this PR to any issues it closes
• New files contain a SPDX license header

[codecov]

Codecov Report

Patch coverage: 86.20% and project coverage change: +0.02 🎉

Comparison is base (9c9a31f) 90.65% compared to head (ca988b4) 90.68%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #346      +/-   ##
==========================================
+ Coverage   90.65%   90.68%   +0.02%     
==========================================
  Files          74       74              
  Lines        6423     6417       -6     
==========================================
- Hits         5823     5819       -4     
+ Misses        600      598       -2     

Impacted Files	Coverage Δ
aquadoggo/src/db/mod.rs	91.66% <ø> (ø)
aquadoggo/src/db/query/order.rs	100.00% <ø> (ø)
aquadoggo/src/db/query/pagination.rs	84.61% <ø> (ø)
aquadoggo/src/db/query/select.rs	96.77% <ø> (ø)
aquadoggo/src/db/query/validate.rs	87.87% <ø> (+1.01%)	⬆️
aquadoggo/src/graphql/scalars/cursor_scalar.rs	73.33% <75.00%> (+15.43%)	⬆️
aquadoggo/src/db/stores/query.rs	94.65% <88.88%> (-0.24%)	⬇️
aquadoggo/src/db/query/filter.rs	90.94% <100.00%> (ø)
aquadoggo/src/graphql/utils.rs	58.91% <100.00%> (ø)

... and 2 files with indirect coverage changes

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