Replace custom cursor encoding with OpaqueCursor

[msk]

This replaces the custom cursor encoding and decoding logic with the OpaqueCursor type provided by async-graphql. This simplifies the code and reduces the risk of errors in cursor handling.

Note that this change breaks backwards compatibility with older versions of the GraphQL API, as the cursor format has changed.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 21 lines in your changes missing coverage. Please review.

Project coverage is 70.38%. Comparing base (238fb77) to head (62cf9fe).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/graphql/cluster.rs	0.00%	7 Missing ⚠️
src/graphql/model.rs	0.00%	7 Missing ⚠️
src/graphql/statistics.rs	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #366      +/-   ##
==========================================
+ Coverage   70.26%   70.38%   +0.11%     
==========================================
  Files          67       67              
  Lines       14400    14376      -24     
==========================================
  Hits        10118    10118              
+ Misses       4282     4258      -24     

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