perf: use a shared resty.Client object for all requests

[kamilon]

This should actually make a noticeable difference in resource usage at TibiaData scale. Back of the napkin math is somewhere around 10MB less GC churn and 10-15% reduced CPU. It should also lower average response time from Tibia.com due to reduced TLS handshakes with the service.

I validated this via build, test and race detector. I also double checked that resty.Client is thread-safe.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 80.76923% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.43%. Comparing base (5203d64) to head (3f7a295).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #644      +/-   ##
==========================================
+ Coverage   78.42%   78.43%   +0.01%     
==========================================
  Files          22       22              
  Lines        3690     3692       +2     
==========================================
+ Hits         2894     2896       +2     
  Misses        658      658              
  Partials      138      138              

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[tobiasehlert]

LGTM