chore: upgrade @koa/router

[stropitek]

Closes: #458

• test: add test for handling unescaped slashes in attachment names
• chore: upgrade @koa/router

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.74%. Comparing base (d822fa7) to head (085558f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #463   +/-   ##
=======================================
  Coverage   71.74%   71.74%           
=======================================
  Files          63       63           
  Lines        3419     3419           
  Branches      705      705           
=======================================
  Hits         2453     2453           
  Misses        840      840           
  Partials      126      126           

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